/*
 * Copyright (c) 2010, 2018, Oracle and/or its affiliates. All rights reserved.
 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
 *
 * This code is free software; you can redistribute it and/or modify it
 * under the terms of the GNU General Public License version 2 only, as
 * published by the Free Software Foundation.  Oracle designates this
 * particular file as subject to the "Classpath" exception as provided
 * by Oracle in the LICENSE file that accompanied this code.
 *
 * This code is distributed in the hope that it will be useful, but WITHOUT
 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
 * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
 * version 2 for more details (a copy is included in the LICENSE file that
 * accompanied this code).
 *
 * You should have received a copy of the GNU General Public License version
 * 2 along with this work; if not, write to the Free Software Foundation,
 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
 *
 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
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 */

package javafx.scene.control.skin;

import com.sun.javafx.scene.ParentHelper;
import com.sun.javafx.scene.control.Logging;
import com.sun.javafx.scene.control.Properties;
import com.sun.javafx.scene.control.VirtualScrollBar;
import com.sun.javafx.scene.control.skin.Utils;
import com.sun.javafx.scene.traversal.Algorithm;
import com.sun.javafx.scene.traversal.Direction;
import com.sun.javafx.scene.traversal.ParentTraversalEngine;
import com.sun.javafx.scene.traversal.TraversalContext;
import javafx.animation.KeyFrame;
import javafx.animation.Timeline;
import javafx.beans.InvalidationListener;
import javafx.beans.Observable;
import javafx.beans.property.BooleanProperty;
import javafx.beans.property.BooleanPropertyBase;
import javafx.beans.property.DoubleProperty;
import javafx.beans.property.IntegerProperty;
import javafx.beans.property.ObjectProperty;
import javafx.beans.property.SimpleBooleanProperty;
import javafx.beans.property.SimpleDoubleProperty;
import javafx.beans.property.SimpleIntegerProperty;
import javafx.beans.property.SimpleObjectProperty;
import javafx.beans.value.ChangeListener;
import javafx.collections.ObservableList;
import javafx.event.EventDispatcher;
import javafx.event.EventHandler;
import javafx.geometry.Orientation;
import javafx.scene.AccessibleRole;
import javafx.scene.Group;
import javafx.scene.Node;
import javafx.scene.Parent;
import javafx.scene.Scene;
import javafx.scene.control.Cell;
import javafx.scene.control.IndexedCell;
import javafx.scene.control.ScrollBar;
import javafx.scene.input.MouseEvent;
import javafx.scene.input.ScrollEvent;
import javafx.scene.layout.Region;
import javafx.scene.layout.StackPane;
import javafx.scene.shape.Rectangle;
import javafx.util.Callback;
import javafx.util.Duration;
import com.sun.javafx.logging.PlatformLogger;

import java.util.AbstractList;
import java.util.ArrayList;
import java.util.BitSet;
import java.util.List;

Implementation of a virtualized container using a cell based mechanism. This is used by the skin implementations for UI controls such as ListView, TreeView, TableView, and TreeTableView. 
Since:
9/**
 * Implementation of a virtualized container using a cell based mechanism. This
 * is used by the skin implementations for UI controls such as
 * {@link javafx.scene.control.ListView}, {@link javafx.scene.control.TreeView},
 * {@link javafx.scene.control.TableView}, and {@link javafx.scene.control.TreeTableView}.
 *
 * @since 9
 */
public class VirtualFlow<T extends IndexedCell> extends Region {

    /***************************************************************************
     *                                                                         *
     * Static fields                                                           *
     *                                                                         *
     **************************************************************************/

    
Scroll events may request to scroll about a number of "lines". We first
decide how big one "line" is - for fixed cell size it's clear,
for variable cell size we settle on a single number so that the scrolling
speed is consistent. Now if the line is so big that
MIN_SCROLLING_LINES_PER_PAGE of them don't fit into one page, we make
them smaller to prevent the scrolling step to be too big (perhaps
even more than one page).
/**
     * Scroll events may request to scroll about a number of "lines". We first
     * decide how big one "line" is - for fixed cell size it's clear,
     * for variable cell size we settle on a single number so that the scrolling
     * speed is consistent. Now if the line is so big that
     * MIN_SCROLLING_LINES_PER_PAGE of them don't fit into one page, we make
     * them smaller to prevent the scrolling step to be too big (perhaps
     * even more than one page).
     */
    private static final int MIN_SCROLLING_LINES_PER_PAGE = 8;

    
Indicates that this is a newly created cell and we need call processCSS for it.
See RT-23616 for more details.
/**
     * Indicates that this is a newly created cell and we need call processCSS for it.
     *
     * See RT-23616 for more details.
     */
    private static final String NEW_CELL = "newcell";

    private static final double GOLDEN_RATIO_MULTIPLIER = 0.618033987;



    
                                                                        *
Private fields                                                          *
                                                                        *
/***************************************************************************
     *                                                                         *
     * Private fields                                                          *
     *                                                                         *
     **************************************************************************/

    private boolean touchDetected = false;
    private boolean mouseDown = false;

    
The width of the VirtualFlow the last time it was laid out. We
use this information for several fast paths during the layout pass.
/**
     * The width of the VirtualFlow the last time it was laid out. We
     * use this information for several fast paths during the layout pass.
     */
    double lastWidth = -1;

    
The height of the VirtualFlow the last time it was laid out. We
use this information for several fast paths during the layout pass.
/**
     * The height of the VirtualFlow the last time it was laid out. We
     * use this information for several fast paths during the layout pass.
     */
    double lastHeight = -1;

    
The number of "virtual" cells in the flow the last time it was laid out.
For example, there may have been 1000 virtual cells, but only 20 actual
cells created and in use. In that case, lastCellCount would be 1000.
/**
     * The number of "virtual" cells in the flow the last time it was laid out.
     * For example, there may have been 1000 virtual cells, but only 20 actual
     * cells created and in use. In that case, lastCellCount would be 1000.
     */
    int lastCellCount = 0;

    
We remember the last value for vertical the last time we laid out the
flow. If vertical has changed, we will want to change the max & value
for the different scroll bars. Since we do all the scroll bar update
work in the layoutChildren function, we need to know what the old value for
vertical was.
/**
     * We remember the last value for vertical the last time we laid out the
     * flow. If vertical has changed, we will want to change the max & value
     * for the different scroll bars. Since we do all the scroll bar update
     * work in the layoutChildren function, we need to know what the old value for
     * vertical was.
     */
    boolean lastVertical;

    
The position last time we laid out. If none of the lastXXX vars have
changed respective to their values in layoutChildren, then we can just punt
out of the method (I hope...)
/**
     * The position last time we laid out. If none of the lastXXX vars have
     * changed respective to their values in layoutChildren, then we can just punt
     * out of the method (I hope...)
     */
    double lastPosition;

    
The breadth of the first visible cell last time we laid out.
/**
     * The breadth of the first visible cell last time we laid out.
     */
    double lastCellBreadth = -1;

    
The length of the first visible cell last time we laid out.
/**
     * The length of the first visible cell last time we laid out.
     */
    double lastCellLength = -1;

    
The list of cells representing those cells which actually make up the
current view. The cells are ordered such that the first cell in this
list is the first in the view, and the last cell is the last in the
view. When pixel scrolling, the list is simply shifted and items drop
off the beginning or the end, depending on the order of scrolling.

This is package private ONLY FOR TESTING
/**
     * The list of cells representing those cells which actually make up the
     * current view. The cells are ordered such that the first cell in this
     * list is the first in the view, and the last cell is the last in the
     * view. When pixel scrolling, the list is simply shifted and items drop
     * off the beginning or the end, depending on the order of scrolling.
     * <p>
     * This is package private ONLY FOR TESTING
     */
    final ArrayLinkedList<T> cells = new ArrayLinkedList<T>();

    
A structure containing cells that can be reused later. These are cells
that at one time were needed to populate the view, but now are no longer
needed. We keep them here until they are needed again.

This is package private ONLY FOR TESTING
/**
     * A structure containing cells that can be reused later. These are cells
     * that at one time were needed to populate the view, but now are no longer
     * needed. We keep them here until they are needed again.
     * <p>
     * This is package private ONLY FOR TESTING
     */
    final ArrayLinkedList<T> pile = new ArrayLinkedList<T>();

    
A special cell used to accumulate bounds, such that we reduce object
churn. This cell must be recreated whenever the cell factory function
changes. This has package access ONLY for testing.
/**
     * A special cell used to accumulate bounds, such that we reduce object
     * churn. This cell must be recreated whenever the cell factory function
     * changes. This has package access ONLY for testing.
     */
    T accumCell;

    
This group is used for holding the 'accumCell'. 'accumCell' must
be added to the skin for it to be styled. Otherwise, it doesn't
report the correct width/height leading to issues when scrolling
the flow
/**
     * This group is used for holding the 'accumCell'. 'accumCell' must
     * be added to the skin for it to be styled. Otherwise, it doesn't
     * report the correct width/height leading to issues when scrolling
     * the flow
     */
    Group accumCellParent;

    
The group which holds the cells.
/**
     * The group which holds the cells.
     */
    final Group sheet;

    final ObservableList<Node> sheetChildren;

    
The scroll bar used for scrolling horizontally. This has package access
ONLY for testing.
/**
     * The scroll bar used for scrolling horizontally. This has package access
     * ONLY for testing.
     */
    private VirtualScrollBar hbar = new VirtualScrollBar(this);

    
The scroll bar used to scrolling vertically. This has package access
ONLY for testing.
/**
     * The scroll bar used to scrolling vertically. This has package access
     * ONLY for testing.
     */
    private VirtualScrollBar vbar = new VirtualScrollBar(this);

    
Control in which the cell's sheet is placed and forms the viewport. The
viewportBreadth and viewportLength are simply the dimensions of the
clipView. This has package access ONLY for testing.
/**
     * Control in which the cell's sheet is placed and forms the viewport. The
     * viewportBreadth and viewportLength are simply the dimensions of the
     * clipView. This has package access ONLY for testing.
     */
    ClippedContainer clipView;

    
When both the horizontal and vertical scroll bars are visible,
we have to 'fill in' the bottom right corner where the two scroll bars
meet. This is handled by this corner region. This has package access
ONLY for testing.
/**
     * When both the horizontal and vertical scroll bars are visible,
     * we have to 'fill in' the bottom right corner where the two scroll bars
     * meet. This is handled by this corner region. This has package access
     * ONLY for testing.
     */
    StackPane corner;

    // used for panning the virtual flow
    private double lastX;
    private double lastY;
    private boolean isPanning = false;

    private boolean fixedCellSizeEnabled = false;

    private boolean needsReconfigureCells = false; // when cell contents are the same
    private boolean needsRecreateCells = false; // when cell factory changed
    private boolean needsRebuildCells = false; // when cell contents have changed
    private boolean needsCellsLayout = false;
    private boolean sizeChanged = false;
    private final BitSet dirtyCells = new BitSet();

    Timeline sbTouchTimeline;
    KeyFrame sbTouchKF1;
    KeyFrame sbTouchKF2;

    private boolean needBreadthBar;
    private boolean needLengthBar;
    private boolean tempVisibility = false;



    /***************************************************************************
     *                                                                         *
     * Constructors                                                            *
     *                                                                         *
     **************************************************************************/

    
Creates a new VirtualFlow instance.
/**
     * Creates a new VirtualFlow instance.
     */
    public VirtualFlow() {
        getStyleClass().add("virtual-flow");
        setId("virtual-flow");

        // initContent
        // --- sheet
        sheet = new Group();
        sheet.getStyleClass().add("sheet");
        sheet.setAutoSizeChildren(false);

        sheetChildren = sheet.getChildren();

        // --- clipView
        clipView = new ClippedContainer(this);
        clipView.setNode(sheet);
        getChildren().add(clipView);

        // --- accumCellParent
        accumCellParent = new Group();
        accumCellParent.setVisible(false);
        getChildren().add(accumCellParent);


        /*
        ** don't allow the ScrollBar to handle the ScrollEvent,
        ** In a VirtualFlow a vertical scroll should scroll on the vertical only,
        ** whereas in a horizontal ScrollBar it can scroll horizontally.
        */
        // block the event from being passed down to children
        final EventDispatcher blockEventDispatcher = (event, tail) -> event;
        // block ScrollEvent from being passed down to scrollbar's skin
        final EventDispatcher oldHsbEventDispatcher = hbar.getEventDispatcher();
        hbar.setEventDispatcher((event, tail) -> {
            if (event.getEventType() == ScrollEvent.SCROLL &&
                    !((ScrollEvent)event).isDirect()) {
                tail = tail.prepend(blockEventDispatcher);
                tail = tail.prepend(oldHsbEventDispatcher);
                return tail.dispatchEvent(event);
            }
            return oldHsbEventDispatcher.dispatchEvent(event, tail);
        });
        // block ScrollEvent from being passed down to scrollbar's skin
        final EventDispatcher oldVsbEventDispatcher = vbar.getEventDispatcher();
        vbar.setEventDispatcher((event, tail) -> {
            if (event.getEventType() == ScrollEvent.SCROLL &&
                    !((ScrollEvent)event).isDirect()) {
                tail = tail.prepend(blockEventDispatcher);
                tail = tail.prepend(oldVsbEventDispatcher);
                return tail.dispatchEvent(event);
            }
            return oldVsbEventDispatcher.dispatchEvent(event, tail);
        });
        /*
        ** listen for ScrollEvents over the whole of the VirtualFlow
        ** area, the above dispatcher having removed the ScrollBars
        ** scroll event handling.
        */
        setOnScroll(new EventHandler<ScrollEvent>() {
            @Override public void handle(ScrollEvent event) {
                if (Properties.IS_TOUCH_SUPPORTED) {
                    if (touchDetected == false &&  mouseDown == false ) {
                        startSBReleasedAnimation();
                    }
                }
                /*
                ** calculate the delta in the direction of the flow.
                */
                double virtualDelta = 0.0;
                if (isVertical()) {
                    switch(event.getTextDeltaYUnits()) {
                        case PAGES:
                            virtualDelta = event.getTextDeltaY() * lastHeight;
                            break;
                        case LINES:
                            double lineSize;
                            if (fixedCellSizeEnabled) {
                                lineSize = getFixedCellSize();
                            } else {
                                // For the scrolling to be reasonably consistent
                                // we set the lineSize to the average size
                                // of all currently loaded lines.
                                T lastCell = cells.getLast();
                                lineSize =
                                        (getCellPosition(lastCell)
                                            + getCellLength(lastCell)
                                            - getCellPosition(cells.getFirst()))
                                        / cells.size();
                            }

                            if (lastHeight / lineSize < MIN_SCROLLING_LINES_PER_PAGE) {
                                lineSize = lastHeight / MIN_SCROLLING_LINES_PER_PAGE;
                            }

                            virtualDelta = event.getTextDeltaY() * lineSize;
                            break;
                        case NONE:
                            virtualDelta = event.getDeltaY();
                    }
                } else { // horizontal
                    switch(event.getTextDeltaXUnits()) {
                        case CHARACTERS:
                            // can we get character size here?
                            // for now, fall through to pixel values
                        case NONE:
                            double dx = event.getDeltaX();
                            double dy = event.getDeltaY();

                            virtualDelta = (Math.abs(dx) > Math.abs(dy) ? dx : dy);
                    }
                }

                if (virtualDelta != 0.0) {
                    /*
                    ** only consume it if we use it
                    */
                    double result = scrollPixels(-virtualDelta);
                    if (result != 0.0) {
                        event.consume();
                    }
                }

                ScrollBar nonVirtualBar = isVertical() ? hbar : vbar;
                if (needBreadthBar) {
                    double nonVirtualDelta = isVertical() ? event.getDeltaX() : event.getDeltaY();
                    if (nonVirtualDelta != 0.0) {
                        double newValue = nonVirtualBar.getValue() - nonVirtualDelta;
                        if (newValue < nonVirtualBar.getMin()) {
                            nonVirtualBar.setValue(nonVirtualBar.getMin());
                        } else if (newValue > nonVirtualBar.getMax()) {
                            nonVirtualBar.setValue(nonVirtualBar.getMax());
                        } else {
                            nonVirtualBar.setValue(newValue);
                        }
                        event.consume();
                    }
                }
            }
        });


        addEventFilter(MouseEvent.MOUSE_PRESSED, new EventHandler<MouseEvent>() {
            @Override
            public void handle(MouseEvent e) {
                mouseDown = true;
                if (Properties.IS_TOUCH_SUPPORTED) {
                    scrollBarOn();
                }
                if (isFocusTraversable()) {
                    // We check here to see if the current focus owner is within
                    // this VirtualFlow, and if so we back-off from requesting
                    // focus back to the VirtualFlow itself. This is particularly
                    // relevant given the bug identified in RT-32869. In this
                    // particular case TextInputControl was clearing selection
                    // when the focus on the TextField changed, meaning that the
                    // right-click context menu was not showing the correct
                    // options as there was no selection in the TextField.
                    boolean doFocusRequest = true;
                    Node focusOwner = getScene().getFocusOwner();
                    if (focusOwner != null) {
                        Parent parent = focusOwner.getParent();
                        while (parent != null) {
                            if (parent.equals(VirtualFlow.this)) {
                                doFocusRequest = false;
                                break;
                            }
                            parent = parent.getParent();
                        }
                    }

                    if (doFocusRequest) {
                        requestFocus();
                    }
                }

                lastX = e.getX();
                lastY = e.getY();

                // determine whether the user has push down on the virtual flow,
                // or whether it is the scrollbar. This is done to prevent
                // mouse events being 'doubled up' when dragging the scrollbar
                // thumb - it has the side-effect of also starting the panning
                // code, leading to flicker
                isPanning = ! (vbar.getBoundsInParent().contains(e.getX(), e.getY())
                        || hbar.getBoundsInParent().contains(e.getX(), e.getY()));
            }
        });
        addEventFilter(MouseEvent.MOUSE_RELEASED, e -> {
            mouseDown = false;
            if (Properties.IS_TOUCH_SUPPORTED) {
                startSBReleasedAnimation();
            }
        });
        addEventFilter(MouseEvent.MOUSE_DRAGGED, e -> {
            if (Properties.IS_TOUCH_SUPPORTED) {
                scrollBarOn();
            }
            if (! isPanning || ! isPannable()) return;

            // With panning enabled, we support panning in both vertical
            // and horizontal directions, regardless of the fact that
            // VirtualFlow is virtual in only one direction.
            double xDelta = lastX - e.getX();
            double yDelta = lastY - e.getY();

            // figure out the distance that the mouse moved in the virtual
            // direction, and then perform the movement along that axis
            // virtualDelta will contain the amount we actually did move
            double virtualDelta = isVertical() ? yDelta : xDelta;
            double actual = scrollPixels(virtualDelta);
            if (actual != 0) {
                // update last* here, as we know we've just adjusted the
                // scrollbar. This means we don't get the situation where a
                // user presses-and-drags a long way past the min or max
                // values, only to change directions and see the scrollbar
                // start moving immediately.
                if (isVertical()) lastY = e.getY();
                else lastX = e.getX();
            }

            // similarly, we do the same in the non-virtual direction
            double nonVirtualDelta = isVertical() ? xDelta : yDelta;
            ScrollBar nonVirtualBar = isVertical() ? hbar : vbar;
            if (nonVirtualBar.isVisible()) {
                double newValue = nonVirtualBar.getValue() + nonVirtualDelta;
                if (newValue < nonVirtualBar.getMin()) {
                    nonVirtualBar.setValue(nonVirtualBar.getMin());
                } else if (newValue > nonVirtualBar.getMax()) {
                    nonVirtualBar.setValue(nonVirtualBar.getMax());
                } else {
                    nonVirtualBar.setValue(newValue);

                    // same as the last* comment above
                    if (isVertical()) lastX = e.getX();
                    else lastY = e.getY();
                }
            }
        });

        /*
         * We place the scrollbars _above_ the rectangle, such that the drag
         * operations often used in conjunction with scrollbars aren't
         * misinterpreted as drag operations on the rectangle as well (which
         * would be the case if the scrollbars were underneath it as the
         * rectangle itself doesn't block the mouse.
         */
        // --- vbar
        vbar.setOrientation(Orientation.VERTICAL);
        vbar.addEventHandler(MouseEvent.ANY, event -> {
            event.consume();
        });
        getChildren().add(vbar);

        // --- hbar
        hbar.setOrientation(Orientation.HORIZONTAL);
        hbar.addEventHandler(MouseEvent.ANY, event -> {
            event.consume();
        });
        getChildren().add(hbar);

        // --- corner
        corner = new StackPane();
        corner.getStyleClass().setAll("corner");
        getChildren().add(corner);



        // initBinds
        // clipView binds
        InvalidationListener listenerX = valueModel -> {
            updateHbar();
        };
        verticalProperty().addListener(listenerX);
        hbar.valueProperty().addListener(listenerX);
        hbar.visibleProperty().addListener(listenerX);

//        ChangeListener listenerY = new ChangeListener() {
//            @Override public void handle(Bean bean, PropertyReference property) {
//                clipView.setClipY(isVertical() ? 0 : vbar.getValue());
//            }
//        };
//        addChangedListener(VERTICAL, listenerY);
//        vbar.addChangedListener(ScrollBar.VALUE, listenerY);

        ChangeListener<Number> listenerY = (ov, t, t1) -> {
            clipView.setClipY(isVertical() ? 0 : vbar.getValue());
        };
        vbar.valueProperty().addListener(listenerY);

        super.heightProperty().addListener((observable, oldHeight, newHeight) -> {
            // Fix for RT-8480, where the VirtualFlow does not show its content
            // after changing size to 0 and back.
            if (oldHeight.doubleValue() == 0 && newHeight.doubleValue() > 0) {
                recreateCells();
            }
        });


        /*
        ** there are certain animations that need to know if the touch is
        ** happening.....
        */
        setOnTouchPressed(e -> {
            touchDetected = true;
            scrollBarOn();
        });

        setOnTouchReleased(e -> {
            touchDetected = false;
            startSBReleasedAnimation();
        });

        ParentHelper.setTraversalEngine(this, new ParentTraversalEngine(this, new Algorithm() {

            Node selectNextAfterIndex(int index, TraversalContext context) {
                T nextCell;
                while ((nextCell = getVisibleCell(++index)) != null) {
                    if (nextCell.isFocusTraversable()) {
                        return nextCell;
                    }
                    Node n = context.selectFirstInParent(nextCell);
                    if (n != null) {
                        return n;
                    }
                }
                return null;
            }

            Node selectPreviousBeforeIndex(int index, TraversalContext context) {
                T prevCell;
                while ((prevCell = getVisibleCell(--index)) != null) {
                    Node prev = context.selectLastInParent(prevCell);
                    if (prev != null) {
                        return prev;
                    }
                    if (prevCell.isFocusTraversable()) {
                        return prevCell;
                    }
                }
                return null;
            }

            @Override
            public Node select(Node owner, Direction dir, TraversalContext context) {
                T cell;
                if (cells.isEmpty()) return null;
                if (cells.contains(owner)) {
                    cell = (T) owner;
                } else {
                    cell = findOwnerCell(owner);
                    Node next = context.selectInSubtree(cell, owner, dir);
                    if (next != null) {
                        return next;
                    }
                    if (dir == Direction.NEXT) dir = Direction.NEXT_IN_LINE;
                }
                int cellIndex = cell.getIndex();
                switch(dir) {
                    case PREVIOUS:
                        return selectPreviousBeforeIndex(cellIndex, context);
                    case NEXT:
                        Node n = context.selectFirstInParent(cell);
                        if (n != null) {
                            return n;
                        }
                        // Intentional fall-through
                    case NEXT_IN_LINE:
                        return selectNextAfterIndex(cellIndex, context);
                }
                return null;
            }

            private T findOwnerCell(Node owner) {
                Parent p = owner.getParent();
                while (!cells.contains(p)) {
                    p = p.getParent();
                }
                return (T)p;
            }

            @Override
            public Node selectFirst(TraversalContext context) {
                T firstCell = cells.getFirst();
                if (firstCell == null) return null;
                if (firstCell.isFocusTraversable()) return firstCell;
                Node n = context.selectFirstInParent(firstCell);
                if (n != null) {
                    return n;
                }
                return selectNextAfterIndex(firstCell.getIndex(), context);
            }

            @Override
            public Node selectLast(TraversalContext context) {
                T lastCell = cells.getLast();
                if (lastCell == null) return null;
                Node p = context.selectLastInParent(lastCell);
                if (p != null) {
                    return p;
                }
                if (lastCell.isFocusTraversable()) return lastCell;
                return selectPreviousBeforeIndex(lastCell.getIndex(), context);
            }
        }));
    }



    /***************************************************************************
     *                                                                         *
     * Properties                                                              *
     *                                                                         *
     **************************************************************************/

    /**
     * There are two main complicating factors in the implementation of the
     * VirtualFlow, which are made even more complicated due to the performance
     * sensitive nature of this code. The first factor is the actual
     * virtualization mechanism, wired together with the PositionMapper.
     * The second complicating factor is the desire to do minimal layout
     * and minimal updates to CSS.
     *
     * Since the layout mechanism runs at most once per pulse, we want to hook
     * into this mechanism for minimal recomputation. Whenever a layout pass
     * is run we record the width/height that the virtual flow was last laid
     * out to. In subsequent passes, if the width/height has not changed then
     * we know we only have to rebuild the cells. If the width or height has
     * changed, then we can make appropriate decisions based on whether the
     * width / height has been reduced or expanded.
     *
     * In various places, if requestLayout is called it is generally just
     * used to indicate that some form of layout needs to happen (either the
     * entire thing has to be reconstructed, or just the cells need to be
     * reconstructed, generally).
     *
     * The accumCell is a special cell which is used in some computations
     * when an actual cell for that item isn't currently available. However,
     * the accumCell must be cleared whenever the cellFactory function is
     * changed because we need to use the cells that come from the new factory.
     *
     * In addition to storing the lastWidth and lastHeight, we also store the
     * number of cells that existed last time we performed a layout. In this
     * way if the number of cells change, we can request a layout and when it
     * occurs we can tell that the number of cells has changed and react
     * accordingly.
     *
     * Because the VirtualFlow can be laid out horizontally or vertically a
     * naming problem is present when trying to conceptualize and implement
     * the flow. In particular, the words "width" and "height" are not
     * precise when describing the unit of measure along the "virtualized"
     * axis and the "orthogonal" axis. For example, the height of a cell when
     * the flow is vertical is the magnitude along the "virtualized axis",
     * and the width is along the axis orthogonal to it.
     *
     * Since "height" and "width" are not reliable terms, we use the words
     * "length" and "breadth" to describe the magnitude of a cell along
     * the virtualized axis and orthogonal axis. For example, in a vertical
     * flow, the height=length and the width=breadth. In a horizontal axis,
     * the height=breadth and the width=length.
     *
     * These terms are somewhat arbitrary, but chosen so that when reading
     * most of the below code you can think in just one dimension, with
     * helper functions converting width/height in to length/breadth, while
     * also being different from width/height so as not to get confused with
     * the actual width/height of a cell.
     */

    // --- vertical
    
Indicates the primary direction of virtualization. If true, then the
primary direction of virtualization is vertical, meaning that cells will
stack vertically on top of each other. If false, then they will stack
horizontally next to each other.
/**
     * Indicates the primary direction of virtualization. If true, then the
     * primary direction of virtualization is vertical, meaning that cells will
     * stack vertically on top of each other. If false, then they will stack
     * horizontally next to each other.
     */
    private BooleanProperty vertical;
    public final void setVertical(boolean value) {
        verticalProperty().set(value);
    }

    public final boolean isVertical() {
        return vertical == null ? true : vertical.get();
    }

    public final BooleanProperty verticalProperty() {
        if (vertical == null) {
            vertical = new BooleanPropertyBase(true) {
                @Override protected void invalidated() {
                    pile.clear();
                    sheetChildren.clear();
                    cells.clear();
                    lastWidth = lastHeight = -1;
                    setMaxPrefBreadth(-1);
                    setViewportBreadth(0);
                    setViewportLength(0);
                    lastPosition = 0;
                    hbar.setValue(0);
                    vbar.setValue(0);
                    setPosition(0.0f);
                    setNeedsLayout(true);
                    requestLayout();
                }

                @Override
                public Object getBean() {
                    return VirtualFlow.this;
                }

                @Override
                public String getName() {
                    return "vertical";
                }
            };
        }
        return vertical;
    }

    // --- pannable
    
Indicates whether the VirtualFlow viewport is capable of being panned
by the user (either via the mouse or touch events).
/**
     * Indicates whether the VirtualFlow viewport is capable of being panned
     * by the user (either via the mouse or touch events).
     */
    private BooleanProperty pannable = new SimpleBooleanProperty(this, "pannable", true);
    public final boolean isPannable() { return pannable.get(); }
    public final void setPannable(boolean value) { pannable.set(value); }
    public final BooleanProperty pannableProperty() { return pannable; }

    // --- cell count
    
Indicates the number of cells that should be in the flow. The user of
the VirtualFlow must set this appropriately. When the cell count changes
the VirtualFlow responds by updating the visuals. If the items backing
the cells change, but the count has not changed, you must call the
reconfigureCells() function to update the visuals.
/**
     * Indicates the number of cells that should be in the flow. The user of
     * the VirtualFlow must set this appropriately. When the cell count changes
     * the VirtualFlow responds by updating the visuals. If the items backing
     * the cells change, but the count has not changed, you must call the
     * reconfigureCells() function to update the visuals.
     */
    private IntegerProperty cellCount = new SimpleIntegerProperty(this, "cellCount", 0) {
        private int oldCount = 0;

        @Override protected void invalidated() {
            int cellCount = get();

            boolean countChanged = oldCount != cellCount;
            oldCount = cellCount;

            // ensure that the virtual scrollbar adjusts in size based on the current
            // cell count.
            if (countChanged) {
                VirtualScrollBar lengthBar = isVertical() ? vbar : hbar;
                lengthBar.setMax(cellCount);
            }

            // I decided *not* to reset maxPrefBreadth here for the following
            // situation. Suppose I have 30 cells and then I add 10 more. Just
            // because I added 10 more doesn't mean the max pref should be
            // reset. Suppose the first 3 cells were extra long, and I was
            // scrolled down such that they weren't visible. If I were to reset
            // maxPrefBreadth when subsequent cells were added or removed, then the
            // scroll bars would erroneously reset as well. So I do not reset
            // the maxPrefBreadth here.

            // Fix for RT-12512, RT-14301 and RT-14864.
            // Without this, the VirtualFlow length-wise scrollbar would not change
            // as expected. This would leave items unable to be shown, as they
            // would exist outside of the visible area, even when the scrollbar
            // was at its maximum position.
            // FIXME this should be only executed on the pulse, so this will likely
            // lead to performance degradation until it is handled properly.
            if (countChanged) {
                layoutChildren();

                // Fix for RT-13965: Without this line of code, the number of items in
                // the sheet would constantly grow, leaking memory for the life of the
                // application. This was especially apparent when the total number of
                // cells changes - regardless of whether it became bigger or smaller.
                sheetChildren.clear();

                Parent parent = getParent();
                if (parent != null) parent.requestLayout();
            }
            // TODO suppose I had 100 cells and I added 100 more. Further
            // suppose I was scrolled to the bottom when that happened. I
            // actually want to update the position of the mapper such that
            // the view remains "stable".
        }
    };
    public final int getCellCount() { return cellCount.get(); }
    public final void setCellCount(int value) { cellCount.set(value);  }
    public final IntegerProperty cellCountProperty() { return cellCount; }


    // --- position
    
The position of the VirtualFlow within its list of cells. This is a value
between 0 and 1.
/**
     * The position of the VirtualFlow within its list of cells. This is a value
     * between 0 and 1.
     */
    private DoubleProperty position = new SimpleDoubleProperty(this, "position") {
        @Override public void setValue(Number v) {
            super.setValue(com.sun.javafx.util.Utils.clamp(0, get(), 1));
        }

        @Override protected void invalidated() {
            super.invalidated();
            requestLayout();
        }
    };
    public final double getPosition() { return position.get(); }
    public final void setPosition(double value) { position.set(value); }
    public final DoubleProperty positionProperty() { return position; }

    // --- fixed cell size
    
For optimisation purposes, some use cases can trade dynamic cell length
for speed - if fixedCellSize is greater than zero we'll use that rather
than determine it by querying the cell itself.
/**
     * For optimisation purposes, some use cases can trade dynamic cell length
     * for speed - if fixedCellSize is greater than zero we'll use that rather
     * than determine it by querying the cell itself.
     */
    private DoubleProperty fixedCellSize = new SimpleDoubleProperty(this, "fixedCellSize") {
        @Override protected void invalidated() {
            fixedCellSizeEnabled = get() > 0;
            needsCellsLayout = true;
            layoutChildren();
        }
    };
    public final void setFixedCellSize(final double value) { fixedCellSize.set(value); }
    public final double getFixedCellSize() { return fixedCellSize.get(); }
    public final DoubleProperty fixedCellSizeProperty() { return fixedCellSize; }


    // --- Cell Factory
    private ObjectProperty<Callback<VirtualFlow<T>, T>> cellFactory;

    
Sets a new cell factory to use in the VirtualFlow. This forces all old
cells to be thrown away, and new cells to be created with
the new cell factory.
Params:
value – the new cell factory/**
     * Sets a new cell factory to use in the VirtualFlow. This forces all old
     * cells to be thrown away, and new cells to be created with
     * the new cell factory.
     * @param value the new cell factory
     */
    public final void setCellFactory(Callback<VirtualFlow<T>, T> value) {
        cellFactoryProperty().set(value);
    }

    
Returns the current cell factory.
Returns:
the current cell factory/**
     * Returns the current cell factory.
     * @return the current cell factory
     */
    public final Callback<VirtualFlow<T>, T> getCellFactory() {
        return cellFactory == null ? null : cellFactory.get();
    }

    
Setting a custom cell factory has the effect of deferring all cell
creation, allowing for total customization of the cell. Internally, the
VirtualFlow is responsible for reusing cells - all that is necessary
is for the custom cell factory to return from this function a cell
which might be usable for representing any item in the VirtualFlow.
Refer to the Cell class documentation for more detail. 
Returns:
 the cell factory property/**
     * <p>Setting a custom cell factory has the effect of deferring all cell
     * creation, allowing for total customization of the cell. Internally, the
     * VirtualFlow is responsible for reusing cells - all that is necessary
     * is for the custom cell factory to return from this function a cell
     * which might be usable for representing any item in the VirtualFlow.
     *
     * <p>Refer to the {@link Cell} class documentation for more detail.
     * @return  the cell factory property
     */
    public final ObjectProperty<Callback<VirtualFlow<T>, T>> cellFactoryProperty() {
        if (cellFactory == null) {
            cellFactory = new SimpleObjectProperty<Callback<VirtualFlow<T>, T>>(this, "cellFactory") {
                @Override protected void invalidated() {
                    if (get() != null) {
                        accumCell = null;
                        setNeedsLayout(true);
                        recreateCells();
                        if (getParent() != null) getParent().requestLayout();
                    }
                }
            };
        }
        return cellFactory;
    }



    /***************************************************************************
     *                                                                         *
     * Public API                                                              *
     *                                                                         *
     **************************************************************************/

    
Overridden to implement somewhat more efficient support for layout. The
VirtualFlow can generally be considered as being unmanaged, in that
whenever the position changes, or other such things change, we need
to perform a layout but there is no reason to notify the parent. However
when things change which may impact the preferred size (such as
vertical, createCell, and configCell) then we need to notify the
parent.
/**
     * Overridden to implement somewhat more efficient support for layout. The
     * VirtualFlow can generally be considered as being unmanaged, in that
     * whenever the position changes, or other such things change, we need
     * to perform a layout but there is no reason to notify the parent. However
     * when things change which may impact the preferred size (such as
     * vertical, createCell, and configCell) then we need to notify the
     * parent.
     */
    @Override public void requestLayout() {
// Note: This block is commented as it was relaying on a bad assumption on how
//       layout request was handled in parent class that is now fixed.
//
//        // isNeedsLayout() is commented out due to RT-21417. This does not
//        // appear to impact performance (indeed, it may help), and resolves the
//        // issue identified in RT-21417.
//        setNeedsLayout(true);

        // The fix is to prograte this layout request to its parent class.
        // A better fix will be required if performance is negatively affected
        // by this fix.
        super.requestLayout();
    }

    
{@inheritDoc} /** {@inheritDoc} */
    @Override protected void layoutChildren() {
        if (needsRecreateCells) {
            lastWidth = -1;
            lastHeight = -1;
            releaseCell(accumCell);
//            accumCell = null;
//            accumCellParent.getChildren().clear();
            sheet.getChildren().clear();
            for (int i = 0, max = cells.size(); i < max; i++) {
                cells.get(i).updateIndex(-1);
            }
            cells.clear();
            pile.clear();
            releaseAllPrivateCells();
        } else if (needsRebuildCells) {
            lastWidth = -1;
            lastHeight = -1;
            releaseCell(accumCell);
            for (int i = 0, max = cells.size(); i < max; i++) {
                cells.get(i).updateIndex(-1);
            }
            addAllToPile();
            releaseAllPrivateCells();
        } else if (needsReconfigureCells) {
            setMaxPrefBreadth(-1);
            lastWidth = -1;
            lastHeight = -1;
        }

        if (! dirtyCells.isEmpty()) {
            int index;
            final int cellsSize = cells.size();
            while ((index = dirtyCells.nextSetBit(0)) != -1 && index < cellsSize) {
                T cell = cells.get(index);
                // updateIndex(-1) works for TableView, but breaks ListView.
                // For now, the TableView just does not use the dirtyCells API
//                cell.updateIndex(-1);
                if (cell != null) {
                    cell.requestLayout();
                }
                dirtyCells.clear(index);
            }

            setMaxPrefBreadth(-1);
            lastWidth = -1;
            lastHeight = -1;
        }

        final boolean hasSizeChange = sizeChanged;
        boolean recreatedOrRebuilt = needsRebuildCells || needsRecreateCells || sizeChanged;

        needsRecreateCells = false;
        needsReconfigureCells = false;
        needsRebuildCells = false;
        sizeChanged = false;

        if (needsCellsLayout) {
            for (int i = 0, max = cells.size(); i < max; i++) {
                Cell<?> cell = cells.get(i);
                if (cell != null) {
                    cell.requestLayout();
                }
            }
            needsCellsLayout = false;

            // yes, we return here - if needsCellsLayout was set to true, we
            // only did it to do the above - not rerun the entire layout.
            return;
        }

        final double width = getWidth();
        final double height = getHeight();
        final boolean isVertical = isVertical();
        final double position = getPosition();

        // if the width and/or height is 0, then there is no point doing
        // any of this work. In particular, this can happen during startup
        if (width <= 0 || height <= 0) {
            addAllToPile();
            lastWidth = width;
            lastHeight = height;
            hbar.setVisible(false);
            vbar.setVisible(false);
            corner.setVisible(false);
            return;
        }

        // we check if any of the cells in the cells list need layout. This is a
        // sign that they are perhaps animating their sizes. Without this check,
        // we may not perform a layout here, meaning that the cell will likely
        // 'jump' (in height normally) when the user drags the virtual thumb as
        // that is the first time the layout would occur otherwise.
        boolean cellNeedsLayout = false;
        boolean thumbNeedsLayout = false;

        if (Properties.IS_TOUCH_SUPPORTED) {
            if ((tempVisibility == true && (hbar.isVisible() == false || vbar.isVisible() == false)) ||
                (tempVisibility == false && (hbar.isVisible() == true || vbar.isVisible() == true))) {
                thumbNeedsLayout = true;
            }
        }

        if (!cellNeedsLayout) {
            for (int i = 0; i < cells.size(); i++) {
                Cell<?> cell = cells.get(i);
                cellNeedsLayout = cell.isNeedsLayout();
                if (cellNeedsLayout) break;
            }
        }

        final int cellCount = getCellCount();
        final T firstCell = getFirstVisibleCell();

        // If no cells need layout, we check other criteria to see if this
        // layout call is even necessary. If it is found that no layout is
        // needed, we just punt.
        if (! cellNeedsLayout && !thumbNeedsLayout) {
            boolean cellSizeChanged = false;
            if (firstCell != null) {
                double breadth = getCellBreadth(firstCell);
                double length = getCellLength(firstCell);
                cellSizeChanged = (breadth != lastCellBreadth) || (length != lastCellLength);
                lastCellBreadth = breadth;
                lastCellLength = length;
            }

            if (width == lastWidth &&
                height == lastHeight &&
                cellCount == lastCellCount &&
                isVertical == lastVertical &&
                position == lastPosition &&
                ! cellSizeChanged)
            {
                // TODO this happens to work around the problem tested by
                // testCellLayout_LayoutWithoutChangingThingsUsesCellsInSameOrderAsBefore
                // but isn't a proper solution. Really what we need to do is, when
                // laying out cells, we need to make sure that if a cell is pressed
                // AND we are doing a full rebuild then we need to make sure we
                // use that cell in the same physical location as before so that
                // it gets the mouse release event.
                return;
            }
        }

        /*
         * This function may get called under a variety of circumstances.
         * It will determine what has changed from the last time it was laid
         * out, and will then take one of several execution paths based on
         * what has changed so as to perform minimal layout work and also to
         * give the expected behavior. One or more of the following may have
         * happened:
         *
         *  1) width/height has changed
         *      - If the width and/or height has been reduced (but neither of
         *        them has been expanded), then we simply have to reposition and
         *        resize the scroll bars
         *      - If the width (in the vertical case) has expanded, then we
         *        need to resize the existing cells and reposition and resize
         *        the scroll bars
         *      - If the height (in the vertical case) has expanded, then we
         *        need to resize and reposition the scroll bars and add
         *        any trailing cells
         *
         *  2) cell count has changed
         *      - If the number of cells is bigger, or it is smaller but not
         *        so small as to move the position then we can just update the
         *        cells in place without performing layout and update the
         *        scroll bars.
         *      - If the number of cells has been reduced and it affects the
         *        position, then move the position and rebuild all the cells
         *        and update the scroll bars
         *
         *  3) size of the cell has changed
         *      - If the size changed in the virtual direction (ie: height
         *        in the case of vertical) then layout the cells, adding
         *        trailing cells as necessary and updating the scroll bars
         *      - If the size changed in the non virtual direction (ie: width
         *        in the case of vertical) then simply adjust the widths of
         *        the cells as appropriate and adjust the scroll bars
         *
         *  4) vertical changed, cells is empty, maxPrefBreadth == -1, etc
         *      - Full rebuild.
         *
         * Each of the conditions really resolves to several of a handful of
         * possible outcomes:
         *  a) reposition & rebuild scroll bars
         *  b) resize cells in non-virtual direction
         *  c) add trailing cells
         *  d) update cells
         *  e) resize cells in the virtual direction
         *  f) all of the above
         *
         * So this function first determines what outcomes need to occur, and
         * then will execute all the ones that really need to happen. Every code
         * path ends up touching the "reposition & rebuild scroll bars" outcome,
         * so that one will be executed every time.
         */
        boolean needTrailingCells = false;
        boolean rebuild = cellNeedsLayout  ||
                isVertical != lastVertical ||
                cells.isEmpty()            ||
                getMaxPrefBreadth() == -1  ||
                position != lastPosition   ||
                cellCount != lastCellCount ||
                hasSizeChange ||
                (isVertical && height < lastHeight) || (! isVertical && width < lastWidth);

        if (!rebuild) {
            // Check if maxPrefBreadth didn't change
            double maxPrefBreadth = getMaxPrefBreadth();
            boolean foundMax = false;
            for (int i = 0; i < cells.size(); ++i) {
                double breadth = getCellBreadth(cells.get(i));
                if (maxPrefBreadth == breadth) {
                    foundMax = true;
                } else if (breadth > maxPrefBreadth) {
                    rebuild = true;
                    break;
                }
            }
            if (!foundMax) { // All values were lower
                rebuild = true;
            }
        }

        if (! rebuild) {
            if ((isVertical && height > lastHeight) || (! isVertical && width > lastWidth)) {
                // resized in the virtual direction
                needTrailingCells = true;
            }
        }

        initViewport();

        // Get the index of the "current" cell
        int currentIndex = computeCurrentIndex();
        if (lastCellCount != cellCount) {
            // The cell count has changed. We want to keep the viewport
            // stable if possible. If position was 0 or 1, we want to keep
            // the position in the same place. If the new cell count is >=
            // the currentIndex, then we will adjust the position to be 1.
            // Otherwise, our goal is to leave the index of the cell at the
            // top consistent, with the same translation etc.
            if (position == 0 || position == 1) {
                // Update the item count
//                setItemCount(cellCount);
            } else if (currentIndex >= cellCount) {
                setPosition(1.0f);
//                setItemCount(cellCount);
            } else if (firstCell != null) {
                double firstCellOffset = getCellPosition(firstCell);
                int firstCellIndex = getCellIndex(firstCell);
//                setItemCount(cellCount);
                adjustPositionToIndex(firstCellIndex);
                double viewportTopToCellTop = -computeOffsetForCell(firstCellIndex);
                adjustByPixelAmount(viewportTopToCellTop - firstCellOffset);
            }

            // Update the current index
            currentIndex = computeCurrentIndex();
        }

        if (rebuild) {
            setMaxPrefBreadth(-1);
            // Start by dumping all the cells into the pile
            addAllToPile();

            // The distance from the top of the viewport to the top of the
            // cell for the current index.
            double offset = -computeViewportOffset(getPosition());

            // Add all the leading and trailing cells (the call to add leading
            // cells will add the current cell as well -- that is, the one that
            // represents the current position on the mapper).
            addLeadingCells(currentIndex, offset);

            // Force filling of space with empty cells if necessary
            addTrailingCells(true);
        } else if (needTrailingCells) {
            addTrailingCells(true);
        }

        computeBarVisiblity();

        recreatedOrRebuilt = recreatedOrRebuilt || rebuild;
        updateScrollBarsAndCells(recreatedOrRebuilt);

        lastWidth = getWidth();
        lastHeight = getHeight();
        lastCellCount = getCellCount();
        lastVertical = isVertical();
        lastPosition = getPosition();

        cleanPile();
    }

    
{@inheritDoc} /** {@inheritDoc} */
    @Override protected void setWidth(double value) {
        if (value != lastWidth) {
            super.setWidth(value);
            sizeChanged = true;
            setNeedsLayout(true);
            requestLayout();
        }
    }

    
{@inheritDoc} /** {@inheritDoc} */
    @Override protected void setHeight(double value) {
        if (value != lastHeight) {
            super.setHeight(value);
            sizeChanged = true;
            setNeedsLayout(true);
            requestLayout();
        }
    }

    
Get a cell which can be used in the layout. This function will reuse
cells from the pile where possible, and will create new cells when
necessary.
Params:
prefIndex – the preferred index
Returns:
the available cell/**
     * Get a cell which can be used in the layout. This function will reuse
     * cells from the pile where possible, and will create new cells when
     * necessary.
     * @param prefIndex the preferred index
     * @return the available cell
     */
    protected T getAvailableCell(int prefIndex) {
        T cell = null;

        // Fix for RT-12822. We try to retrieve the cell from the pile rather
        // than just grab a random cell from the pile (or create another cell).
        for (int i = 0, max = pile.size(); i < max; i++) {
            T _cell = pile.get(i);
            assert _cell != null;

            if (getCellIndex(_cell) == prefIndex) {
                cell = _cell;
                pile.remove(i);
                break;
            }
        }

        if (cell == null && !pile.isEmpty()) {
            cell = pile.removeLast();
        }

        if (cell == null) {
            cell = getCellFactory().call(this);
            cell.getProperties().put(NEW_CELL, null);
        }

        if (cell.getParent() == null) {
            sheetChildren.add(cell);
        }

        return cell;
    }

    
This method will remove all cells from the VirtualFlow and remove them,
adding them to the 'pile' (that is, a place from where cells can be used
at a later date). This method is protected to allow subclasses to clean up
appropriately.
/**
     * This method will remove all cells from the VirtualFlow and remove them,
     * adding them to the 'pile' (that is, a place from where cells can be used
     * at a later date). This method is protected to allow subclasses to clean up
     * appropriately.
     */
    protected void addAllToPile() {
        for (int i = 0, max = cells.size(); i < max; i++) {
            addToPile(cells.removeFirst());
        }
    }

    
Gets a cell for the given index if the cell has been created and laid out.
"Visible" is a bit of a misnomer, the cell might not be visible in the
viewport (it may be clipped), but does distinguish between cells that
have been created and are in use vs. those that are in the pile or
not created.
Params:
index – the index
Returns:
the visible cell/**
     * Gets a cell for the given index if the cell has been created and laid out.
     * "Visible" is a bit of a misnomer, the cell might not be visible in the
     * viewport (it may be clipped), but does distinguish between cells that
     * have been created and are in use vs. those that are in the pile or
     * not created.
     * @param index the index
     * @return the visible cell
     */
    public T getVisibleCell(int index) {
        if (cells.isEmpty()) return null;

        // check the last index
        T lastCell = cells.getLast();
        int lastIndex = getCellIndex(lastCell);
        if (index == lastIndex) return lastCell;

        // check the first index
        T firstCell = cells.getFirst();
        int firstIndex = getCellIndex(firstCell);
        if (index == firstIndex) return firstCell;

        // if index is > firstIndex and < lastIndex then we can get the index
        if (index > firstIndex && index < lastIndex) {
            T cell = cells.get(index - firstIndex);
            if (getCellIndex(cell) == index) return cell;
        }

        // there is no visible cell for the specified index
        return null;
    }

    
Locates and returns the last non-empty IndexedCell that is currently
partially or completely visible. This function may return null if there
are no cells, or if the viewport length is 0.
Returns:
the last visible cell/**
     * Locates and returns the last non-empty IndexedCell that is currently
     * partially or completely visible. This function may return null if there
     * are no cells, or if the viewport length is 0.
     * @return the last visible cell
     */
    public T getLastVisibleCell() {
        if (cells.isEmpty() || getViewportLength() <= 0) return null;

        T cell;
        for (int i = cells.size() - 1; i >= 0; i--) {
            cell = cells.get(i);
            if (! cell.isEmpty()) {
                return cell;
            }
        }

        return null;
    }

    
Locates and returns the first non-empty IndexedCell that is partially or
completely visible. This really only ever returns null if there are no
cells or the viewport length is 0.
Returns:
the first visible cell/**
     * Locates and returns the first non-empty IndexedCell that is partially or
     * completely visible. This really only ever returns null if there are no
     * cells or the viewport length is 0.
     * @return the first visible cell
     */
    public T getFirstVisibleCell() {
        if (cells.isEmpty() || getViewportLength() <= 0) return null;
        T cell = cells.getFirst();
        return cell.isEmpty() ? null : cell;
    }

    
Adjust the position of cells so that the specified cell
will be positioned at the start of the viewport. The given cell must
already be "live".
Params:
firstCell – the first cell/**
     * Adjust the position of cells so that the specified cell
     * will be positioned at the start of the viewport. The given cell must
     * already be "live".
     * @param firstCell the first cell
     */
    public void scrollToTop(T firstCell) {
        if (firstCell != null) {
            scrollPixels(getCellPosition(firstCell));
        }
    }

    
Adjust the position of cells so that the specified cell
will be positioned at the end of the viewport. The given cell must
already be "live".
Params:
lastCell – the last cell/**
     * Adjust the position of cells so that the specified cell
     * will be positioned at the end of the viewport. The given cell must
     * already be "live".
     * @param lastCell the last cell
     */
    public void scrollToBottom(T lastCell) {
        if (lastCell != null) {
            scrollPixels(getCellPosition(lastCell) + getCellLength(lastCell) - getViewportLength());
        }
    }

    
Adjusts the cells such that the selected cell will be fully visible in
the viewport (but only just).
Params:
cell – the cell/**
     * Adjusts the cells such that the selected cell will be fully visible in
     * the viewport (but only just).
     * @param cell the cell
     */
    public void scrollTo(T cell) {
        if (cell != null) {
            final double start = getCellPosition(cell);
            final double length = getCellLength(cell);
            final double end = start + length;
            final double viewportLength = getViewportLength();

            if (start < 0) {
                scrollPixels(start);
            } else if (end > viewportLength) {
                scrollPixels(end - viewportLength);
            }
        }
    }

    
Adjusts the cells such that the cell in the given index will be fully visible in
the viewport.
Params:
index – the index/**
     * Adjusts the cells such that the cell in the given index will be fully visible in
     * the viewport.
     * @param index the index
     */
    public void scrollTo(int index) {
        T cell = getVisibleCell(index);
        if (cell != null) {
            scrollTo(cell);
        } else {
            adjustPositionToIndex(index);
            addAllToPile();
            requestLayout();
        }
    }

    
Adjusts the cells such that the cell in the given index will be fully visible in
the viewport, and positioned at the very top of the viewport.
Params:
index – the index/**
     * Adjusts the cells such that the cell in the given index will be fully visible in
     * the viewport, and positioned at the very top of the viewport.
     * @param index the index
     */
    public void scrollToTop(int index) {
        boolean posSet = false;

        if (index >= getCellCount() - 1) {
            setPosition(1);
            posSet = true;
        } else if (index < 0) {
            setPosition(0);
            posSet = true;
        }

        if (! posSet) {
            adjustPositionToIndex(index);
            double offset = - computeOffsetForCell(index);
            adjustByPixelAmount(offset);
        }

        requestLayout();
    }

//    //TODO We assume all the cell have the same length.  We will need to support
//    // cells of different lengths.
//    public void scrollToOffset(int offset) {
//        scrollPixels(offset * getCellLength(0));
//    }

    
Given a delta value representing a number of pixels, this method attempts
to move the VirtualFlow in the given direction (positive is down/right,
negative is up/left) the given number of pixels. It returns the number of
pixels actually moved.
Params:
delta – the delta value
Returns:
the number of pixels actually moved/**
     * Given a delta value representing a number of pixels, this method attempts
     * to move the VirtualFlow in the given direction (positive is down/right,
     * negative is up/left) the given number of pixels. It returns the number of
     * pixels actually moved.
     * @param delta the delta value
     * @return the number of pixels actually moved
     */
    public double scrollPixels(final double delta) {
        // Short cut this method for cases where nothing should be done
        if (delta == 0) return 0;

        final boolean isVertical = isVertical();
        if (((isVertical && (tempVisibility ? !needLengthBar : !vbar.isVisible())) ||
                (! isVertical && (tempVisibility ? !needLengthBar : !hbar.isVisible())))) return 0;

        double pos = getPosition();
        if (pos == 0.0f && delta < 0) return 0;
        if (pos == 1.0f && delta > 0) return 0;

        adjustByPixelAmount(delta);
        if (pos == getPosition()) {
            // The pos hasn't changed, there's nothing to do. This is likely
            // to occur when we hit either extremity
            return 0;
        }

        // Now move stuff around. Translating by pixels fundamentally means
        // moving the cells by the delta. However, after having
        // done that, we need to go through the cells and see which cells,
        // after adding in the translation factor, now fall off the viewport.
        // Also, we need to add cells as appropriate to the end (or beginning,
        // depending on the direction of travel).
        //
        // One simplifying assumption (that had better be true!) is that we
        // will only make it this far in the function if the virtual scroll
        // bar is visible. Otherwise, we never will pixel scroll. So as we go,
        // if we find that the maxPrefBreadth exceeds the viewportBreadth,
        // then we will be sure to show the breadthBar and update it
        // accordingly.
        if (cells.size() > 0) {
            for (int i = 0; i < cells.size(); i++) {
                T cell = cells.get(i);
                assert cell != null;
                positionCell(cell, getCellPosition(cell) - delta);
            }

            // Fix for RT-32908
            T firstCell = cells.getFirst();
            double layoutY = firstCell == null ? 0 : getCellPosition(firstCell);
            for (int i = 0; i < cells.size(); i++) {
                T cell = cells.get(i);
                assert cell != null;
                double actualLayoutY = getCellPosition(cell);
                if (Math.abs(actualLayoutY - layoutY) > 0.001) {
                    // we need to shift the cell to layoutY
                    positionCell(cell, layoutY);
                }

                layoutY += getCellLength(cell);
            }
            // end of fix for RT-32908
            cull();
            firstCell = cells.getFirst();

            // Add any necessary leading cells
            if (firstCell != null) {
                int firstIndex = getCellIndex(firstCell);
                double prevIndexSize = getCellLength(firstIndex - 1);
                addLeadingCells(firstIndex - 1, getCellPosition(firstCell) - prevIndexSize);
            } else {
                int currentIndex = computeCurrentIndex();

                // The distance from the top of the viewport to the top of the
                // cell for the current index.
                double offset = -computeViewportOffset(getPosition());

                // Add all the leading and trailing cells (the call to add leading
                // cells will add the current cell as well -- that is, the one that
                // represents the current position on the mapper).
                addLeadingCells(currentIndex, offset);
            }

            // Starting at the tail of the list, loop adding cells until
            // all the space on the table is filled up. We want to make
            // sure that we DO NOT add empty trailing cells (since we are
            // in the full virtual case and so there are no trailing empty
            // cells).
            if (! addTrailingCells(false)) {
                // Reached the end, but not enough cells to fill up to
                // the end. So, remove the trailing empty space, and translate
                // the cells down
                final T lastCell = getLastVisibleCell();
                final double lastCellSize = getCellLength(lastCell);
                final double cellEnd = getCellPosition(lastCell) + lastCellSize;
                final double viewportLength = getViewportLength();

                if (cellEnd < viewportLength) {
                    // Reposition the nodes
                    double emptySize = viewportLength - cellEnd;
                    for (int i = 0; i < cells.size(); i++) {
                        T cell = cells.get(i);
                        positionCell(cell, getCellPosition(cell) + emptySize);
                    }
                    setPosition(1.0f);
                    // fill the leading empty space
                    firstCell = cells.getFirst();
                    int firstIndex = getCellIndex(firstCell);
                    double prevIndexSize = getCellLength(firstIndex - 1);
                    addLeadingCells(firstIndex - 1, getCellPosition(firstCell) - prevIndexSize);
                }
            }
        }

        // Now throw away any cells that don't fit
        cull();

        // Finally, update the scroll bars
        updateScrollBarsAndCells(false);
        lastPosition = getPosition();

        // notify
        return delta; // TODO fake
    }

    
{@inheritDoc} /** {@inheritDoc} */
    @Override protected double computePrefWidth(double height) {
        double w = isVertical() ? getPrefBreadth(height) : getPrefLength();
        return w + vbar.prefWidth(-1);
    }

    
{@inheritDoc} /** {@inheritDoc} */
    @Override protected double computePrefHeight(double width) {
        double h = isVertical() ? getPrefLength() : getPrefBreadth(width);
        return h + hbar.prefHeight(-1);
    }

    
Return a cell for the given index. This may be called for any cell,
including beyond the range defined by cellCount, in which case an
empty cell will be returned. The returned value should not be stored for
any reason.
Params:
index – the index
Returns:
the cell/**
     * Return a cell for the given index. This may be called for any cell,
     * including beyond the range defined by cellCount, in which case an
     * empty cell will be returned. The returned value should not be stored for
     * any reason.
     * @param index the index
     * @return the cell
     */
    public T getCell(int index) {
        // If there are cells, then we will attempt to get an existing cell
        if (! cells.isEmpty()) {
            // First check the cells that have already been created and are
            // in use. If this call returns a value, then we can use it
            T cell = getVisibleCell(index);
            if (cell != null) return cell;
        }

        // check the pile
        for (int i = 0; i < pile.size(); i++) {
            T cell = pile.get(i);
            if (getCellIndex(cell) == index) {
                // Note that we don't remove from the pile: if we do it leads
                // to a severe performance decrease. This seems to be OK, as
                // getCell() is only used for cell measurement purposes.
                // pile.remove(i);
                return cell;
            }
        }

        if (pile.size() > 0) {
            return pile.get(0);
        }

        // We need to use the accumCell and return that
        if (accumCell == null) {
            Callback<VirtualFlow<T>,T> cellFactory = getCellFactory();
            if (cellFactory != null) {
                accumCell = cellFactory.call(this);
                accumCell.getProperties().put(NEW_CELL, null);
                accumCellParent.getChildren().setAll(accumCell);

                // Note the screen reader will attempt to find all
                // the items inside the view to calculate the item count.
                // Having items under different parents (sheet and accumCellParent)
                // leads the screen reader to compute wrong values.
                // The regular scheme to provide items to the screen reader
                // uses getPrivateCell(), which places the item in the sheet.
                // The accumCell, and its children, should be ignored by the
                // screen reader.
                accumCell.setAccessibleRole(AccessibleRole.NODE);
                accumCell.getChildrenUnmodifiable().addListener((Observable c) -> {
                    for (Node n : accumCell.getChildrenUnmodifiable()) {
                        n.setAccessibleRole(AccessibleRole.NODE);
                    }
                });
            }
        }
        setCellIndex(accumCell, index);
        resizeCellSize(accumCell);
        return accumCell;
    }

    
The VirtualFlow uses this method to set a cells index (rather than calling IndexedCell.updateIndex(int) directly), so it is a perfect place for subclasses to override if this if of interest. 
Params:
cell – The cell whose index will be updated.
index – The new index for the cell./**
     * The VirtualFlow uses this method to set a cells index (rather than calling
     * {@link IndexedCell#updateIndex(int)} directly), so it is a perfect place
     * for subclasses to override if this if of interest.
     *
     * @param cell The cell whose index will be updated.
     * @param index The new index for the cell.
     */
    protected void setCellIndex(T cell, int index) {
        assert cell != null;

        cell.updateIndex(index);

        // make sure the cell is sized correctly. This is important for both
        // general layout of cells in a VirtualFlow, but also in cases such as
        // RT-34333, where the sizes were being reported incorrectly to the
        // ComboBox popup.
        if ((cell.isNeedsLayout() && cell.getScene() != null) || cell.getProperties().containsKey(NEW_CELL)) {
            cell.applyCss();
            cell.getProperties().remove(NEW_CELL);
        }
    }

    
Return the index for a given cell. This allows subclasses to customise
how cell indices are retrieved.
Params:
cell – the cell
Returns:
the index/**
     * Return the index for a given cell. This allows subclasses to customise
     * how cell indices are retrieved.
     * @param cell the cell
     * @return the index
     */
    protected int getCellIndex(T cell){
        return cell.getIndex();
    }



    
                                                                        *
Private implementation                                                  *
                                                                        *
/***************************************************************************
     *                                                                         *
     * Private implementation                                                  *
     *                                                                         *
     **************************************************************************/

    final VirtualScrollBar getHbar() {
        return hbar;
    }
    final VirtualScrollBar getVbar() {
        return vbar;
    }

    
The maximum preferred size in the non-virtual direction. For example,
if vertical, then this is the max pref width of all cells encountered.

In general, this is the largest preferred size in the non-virtual
direction that we have ever encountered. We don't reduce this size
unless instructed to do so, so as to reduce the amount of scroll bar
jitter. The access on this variable is package ONLY FOR TESTING.
/**
     * The maximum preferred size in the non-virtual direction. For example,
     * if vertical, then this is the max pref width of all cells encountered.
     * <p>
     * In general, this is the largest preferred size in the non-virtual
     * direction that we have ever encountered. We don't reduce this size
     * unless instructed to do so, so as to reduce the amount of scroll bar
     * jitter. The access on this variable is package ONLY FOR TESTING.
     */
    private double maxPrefBreadth;
    private final void setMaxPrefBreadth(double value) {
        this.maxPrefBreadth = value;
    }
    final double getMaxPrefBreadth() {
        return maxPrefBreadth;
    }

    
The breadth of the viewport portion of the VirtualFlow as computed during
the layout pass. In a vertical flow this would be the same as the clip
view width. In a horizontal flow this is the clip view height.
The access on this variable is package ONLY FOR TESTING.
/**
     * The breadth of the viewport portion of the VirtualFlow as computed during
     * the layout pass. In a vertical flow this would be the same as the clip
     * view width. In a horizontal flow this is the clip view height.
     * The access on this variable is package ONLY FOR TESTING.
     */
    private double viewportBreadth;
    private final void setViewportBreadth(double value) {
        this.viewportBreadth = value;
    }
    private final double getViewportBreadth() {
        return viewportBreadth;
    }

    
The length of the viewport portion of the VirtualFlow as computed
during the layout pass. In a vertical flow this would be the same as the
clip view height. In a horizontal flow this is the clip view width.
The access on this variable is package ONLY FOR TESTING.
/**
     * The length of the viewport portion of the VirtualFlow as computed
     * during the layout pass. In a vertical flow this would be the same as the
     * clip view height. In a horizontal flow this is the clip view width.
     * The access on this variable is package ONLY FOR TESTING.
     */
    private double viewportLength;
    void setViewportLength(double value) {
        this.viewportLength = value;
    }
    double getViewportLength() {
        return viewportLength;
    }

    
Compute and return the length of the cell for the given index. This is
called both internally when adjusting by pixels, and also at times
by PositionMapper (see the getItemSize callback). When called by
PositionMapper, it is possible that it will be called for some index
which is not associated with any cell, so we have to do a bit of work
to use a cell as a helper for computing cell size in some cases.
/**
     * Compute and return the length of the cell for the given index. This is
     * called both internally when adjusting by pixels, and also at times
     * by PositionMapper (see the getItemSize callback). When called by
     * PositionMapper, it is possible that it will be called for some index
     * which is not associated with any cell, so we have to do a bit of work
     * to use a cell as a helper for computing cell size in some cases.
     */
    double getCellLength(int index) {
        if (fixedCellSizeEnabled) return getFixedCellSize();

        T cell = getCell(index);
        double length = getCellLength(cell);
        releaseCell(cell);
        return length;
    }

    /**
     */
    double getCellBreadth(int index) {
        T cell = getCell(index);
        double b = getCellBreadth(cell);
        releaseCell(cell);
        return b;
    }

    
Gets the length of a specific cell
/**
     * Gets the length of a specific cell
     */
    double getCellLength(T cell) {
        if (cell == null) return 0;
        if (fixedCellSizeEnabled) return getFixedCellSize();

        return isVertical() ?
                cell.getLayoutBounds().getHeight()
                : cell.getLayoutBounds().getWidth();
    }

    
Gets the breadth of a specific cell
/**
     * Gets the breadth of a specific cell
     */
    double getCellBreadth(Cell cell) {
        return isVertical() ?
                cell.prefWidth(-1)
                : cell.prefHeight(-1);
    }

    
Gets the layout position of the cell along the length axis
/**
     * Gets the layout position of the cell along the length axis
     */
    double getCellPosition(T cell) {
        if (cell == null) return 0;

        return isVertical() ?
                cell.getLayoutY()
                : cell.getLayoutX();
    }

    private void positionCell(T cell, double position) {
        if (isVertical()) {
            cell.setLayoutX(0);
            cell.setLayoutY(snapSizeY(position));
        } else {
            cell.setLayoutX(snapSizeX(position));
            cell.setLayoutY(0);
        }
    }

    private void resizeCellSize(T cell) {
        if (cell == null) return;

        if (isVertical()) {
            double width = Math.max(getMaxPrefBreadth(), getViewportBreadth());
            cell.resize(width, fixedCellSizeEnabled ? getFixedCellSize() : Utils.boundedSize(cell.prefHeight(width), cell.minHeight(width), cell.maxHeight(width)));
        } else {
            double height = Math.max(getMaxPrefBreadth(), getViewportBreadth());
            cell.resize(fixedCellSizeEnabled ? getFixedCellSize() : Utils.boundedSize(cell.prefWidth(height), cell.minWidth(height), cell.maxWidth(height)), height);
        }
    }

    private List<T> getCells() {
        return cells;
    }

    // Returns last visible cell whose bounds are entirely within the viewport
    T getLastVisibleCellWithinViewPort() {
        if (cells.isEmpty() || getViewportLength() <= 0) return null;

        T cell;
        final double max = getViewportLength();
        for (int i = cells.size() - 1; i >= 0; i--) {
            cell = cells.get(i);
            if (cell.isEmpty()) continue;

            final double cellStart = getCellPosition(cell);
            final double cellEnd = cellStart + getCellLength(cell);

            // we use the magic +2 to allow for a little bit of fuzziness,
            // this is to help in situations such as RT-34407
            if (cellEnd <= (max + 2)) {
                return cell;
            }
        }

        return null;
    }

    // Returns first visible cell whose bounds are entirely within the viewport
    T getFirstVisibleCellWithinViewPort() {
        if (cells.isEmpty() || getViewportLength() <= 0) return null;

        T cell;
        for (int i = 0; i < cells.size(); i++) {
            cell = cells.get(i);
            if (cell.isEmpty()) continue;

            final double cellStart = getCellPosition(cell);
            if (cellStart >= 0) {
                return cell;
            }
        }

        return null;
    }

    
Adds all the cells prior to and including the given currentIndex, until
no more can be added without falling off the flow. The startOffset
indicates the distance from the leading edge (top) of the viewport to
the leading edge (top) of the currentIndex.
/**
     * Adds all the cells prior to and including the given currentIndex, until
     * no more can be added without falling off the flow. The startOffset
     * indicates the distance from the leading edge (top) of the viewport to
     * the leading edge (top) of the currentIndex.
     */
    void addLeadingCells(int currentIndex, double startOffset) {
        // The offset will keep track of the distance from the top of the
        // viewport to the top of the current index. We will increment it
        // as we lay out leading cells.
        double offset = startOffset;
        // The index is the absolute index of the cell being laid out
        int index = currentIndex;

        // Offset should really be the bottom of the current index
        boolean first = true; // first time in, we just fudge the offset and let
        // it be the top of the current index then redefine
        // it as the bottom of the current index thereafter
        // while we have not yet laid out so many cells that they would fall
        // off the flow, we will continue to create and add cells. The
        // offset is our indication of whether we can lay out additional
        // cells. If the offset is ever < 0, except in the case of the very
        // first cell, then we must quit.
        T cell = null;

        // special case for the position == 1.0, skip adding last invisible cell
        if (index == getCellCount() && offset == getViewportLength()) {
            index--;
            first = false;
        }
        while (index >= 0 && (offset > 0 || first)) {
            cell = getAvailableCell(index);
            setCellIndex(cell, index);
            resizeCellSize(cell); // resize must be after config
            cells.addFirst(cell);

            // A little gross but better than alternatives because it reduces
            // the number of times we have to update a cell or compute its
            // size. The first time into this loop "offset" is actually the
            // top of the current index. On all subsequent visits, it is the
            // bottom of the current index.
            if (first) {
                first = false;
            } else {
                offset -= getCellLength(cell);
            }

            // Position the cell, and update the maxPrefBreadth variable as we go.
            positionCell(cell, offset);
            setMaxPrefBreadth(Math.max(getMaxPrefBreadth(), getCellBreadth(cell)));
            cell.setVisible(true);
            --index;
        }

        // There are times when after laying out the cells we discover that
        // the top of the first cell which represents index 0 is below the top
        // of the viewport. In these cases, we have to adjust the cells up
        // and reset the mapper position. This might happen when items got
        // removed at the top or when the viewport size increased.
        if (cells.size() > 0) {
            cell = cells.getFirst();
            int firstIndex = getCellIndex(cell);
            double firstCellPos = getCellPosition(cell);
            if (firstIndex == 0 && firstCellPos > 0) {
                setPosition(0.0f);
                offset = 0;
                for (int i = 0; i < cells.size(); i++) {
                    cell = cells.get(i);
                    positionCell(cell, offset);
                    offset += getCellLength(cell);
                }
            }
        } else {
            // reset scrollbar to top, so if the flow sees cells again it starts at the top
            vbar.setValue(0);
            hbar.setValue(0);
        }
    }

    
Adds all the trailing cells that come after the last index in
the cells ObservableList.
/**
     * Adds all the trailing cells that come <em>after</em> the last index in
     * the cells ObservableList.
     */
    boolean addTrailingCells(boolean fillEmptyCells) {
        // If cells is empty then addLeadingCells bailed for some reason and
        // we're hosed, so just punt
        if (cells.isEmpty()) return false;

        // While we have not yet laid out so many cells that they would fall
        // off the flow, so we will continue to create and add cells. When the
        // offset becomes greater than the width/height of the flow, then we
        // know we cannot add any more cells.
        T startCell = cells.getLast();
        double offset = getCellPosition(startCell) + getCellLength(startCell);
        int index = getCellIndex(startCell) + 1;
        final int cellCount = getCellCount();
        boolean filledWithNonEmpty = index <= cellCount;

        final double viewportLength = getViewportLength();

        // Fix for RT-37421, which was a regression caused by RT-36556
        if (offset < 0 && !fillEmptyCells) {
            return false;
        }

        //
        // RT-36507: viewportLength gives the maximum number of
        // additional cells that should ever be able to fit in the viewport if
        // every cell had a height of 1. If index ever exceeds this count,
        // then offset is not incrementing fast enough, or at all, which means
        // there is something wrong with the cell size calculation.
        //
        final double maxCellCount = viewportLength;
        while (offset < viewportLength) {
            if (index >= cellCount) {
                if (offset < viewportLength) filledWithNonEmpty = false;
                if (! fillEmptyCells) return filledWithNonEmpty;
                // RT-36507 - return if we've exceeded the maximum
                if (index > maxCellCount) {
                    final PlatformLogger logger = Logging.getControlsLogger();
                    if (logger.isLoggable(PlatformLogger.Level.INFO)) {
                        logger.info("index exceeds maxCellCount. Check size calculations for " + startCell.getClass());
                    }
                    return filledWithNonEmpty;
                }
            }
            T cell = getAvailableCell(index);
            setCellIndex(cell, index);
            resizeCellSize(cell); // resize happens after config!
            cells.addLast(cell);

            // Position the cell and update the max pref
            positionCell(cell, offset);
            setMaxPrefBreadth(Math.max(getMaxPrefBreadth(), getCellBreadth(cell)));

            offset += getCellLength(cell);
            cell.setVisible(true);
            ++index;
        }

        // Discover whether the first cell coincides with index #0. If after
        // adding all the trailing cells we find that a) the first cell was
        // not index #0 and b) there are trailing cells, then we have a
        // problem. We need to shift all the cells down and add leading cells,
        // one at a time, until either the very last non-empty cells is aligned
        // with the bottom OR we have laid out cell index #0 at the first
        // position.
        T firstCell = cells.getFirst();
        index = getCellIndex(firstCell);
        T lastNonEmptyCell = getLastVisibleCell();
        double start = getCellPosition(firstCell);
        double end = getCellPosition(lastNonEmptyCell) + getCellLength(lastNonEmptyCell);
        if ((index != 0 || (index == 0 && start < 0)) && fillEmptyCells &&
                lastNonEmptyCell != null && getCellIndex(lastNonEmptyCell) == cellCount - 1 && end < viewportLength) {

            double prospectiveEnd = end;
            double distance = viewportLength - end;
            while (prospectiveEnd < viewportLength && index != 0 && (-start) < distance) {
                index--;
                T cell = getAvailableCell(index);
                setCellIndex(cell, index);
                resizeCellSize(cell); // resize must be after config
                cells.addFirst(cell);
                double cellLength = getCellLength(cell);
                start -= cellLength;
                prospectiveEnd += cellLength;
                positionCell(cell, start);
                setMaxPrefBreadth(Math.max(getMaxPrefBreadth(), getCellBreadth(cell)));
                cell.setVisible(true);
            }

            // The amount by which to translate the cells down
            firstCell = cells.getFirst();
            start = getCellPosition(firstCell);
            double delta = viewportLength - end;
            if (getCellIndex(firstCell) == 0 && delta > (-start)) {
                delta = (-start);
            }
            // Move things
            for (int i = 0; i < cells.size(); i++) {
                T cell = cells.get(i);
                positionCell(cell, getCellPosition(cell) + delta);
            }

            // Check whether the first cell, subsequent to our adjustments, is
            // now index #0 and aligned with the top. If so, change the position
            // to be at 0 instead of 1.
            start = getCellPosition(firstCell);
            if (getCellIndex(firstCell) == 0 && start == 0) {
                setPosition(0);
            } else if (getPosition() != 1) {
                setPosition(1);
            }
        }

        return filledWithNonEmpty;
    }

    void reconfigureCells() {
        needsReconfigureCells = true;
        requestLayout();
    }

    void recreateCells() {
        needsRecreateCells = true;
        requestLayout();
    }

    void rebuildCells() {
        needsRebuildCells = true;
        requestLayout();
    }

    void requestCellLayout() {
        needsCellsLayout = true;
        requestLayout();
    }

    void setCellDirty(int index) {
        dirtyCells.set(index);
        requestLayout();
    }

    private void startSBReleasedAnimation() {
        if (sbTouchTimeline == null) {
            /*
            ** timeline to leave the scrollbars visible for a short
            ** while after a scroll/drag
            */
            sbTouchTimeline = new Timeline();
            sbTouchKF1 = new KeyFrame(Duration.millis(0), event -> {
                tempVisibility = true;
                requestLayout();
            });

            sbTouchKF2 = new KeyFrame(Duration.millis(1000), event -> {
                if (touchDetected == false && mouseDown == false) {
                    tempVisibility = false;
                    requestLayout();
                }
            });
            sbTouchTimeline.getKeyFrames().addAll(sbTouchKF1, sbTouchKF2);
        }
        sbTouchTimeline.playFromStart();
    }

    private void scrollBarOn() {
        tempVisibility = true;
        requestLayout();
    }

    void updateHbar() {
        if (! isVisible() || getScene() == null) return;
        // Bring the clipView.clipX back to 0 if control is vertical or
        // the hbar isn't visible (fix for RT-11666)
        if (isVertical()) {
            if (hbar.isVisible()) {
                clipView.setClipX(hbar.getValue());
            } else {
                // all cells are now less than the width of the flow,
                // so we should shift the hbar/clip such that
                // everything is visible in the viewport.
                clipView.setClipX(0);
                hbar.setValue(0);
            }
        }
    }

    
Returns:
true if bar visibility changed/**
     * @return true if bar visibility changed
     */
    private boolean computeBarVisiblity() {
        if (cells.isEmpty()) {
            // In case no cells are set yet, we assume no bars are needed
            needLengthBar = false;
            needBreadthBar = false;
            return true;
        }

        final boolean isVertical = isVertical();
        boolean barVisibilityChanged = false;

        VirtualScrollBar breadthBar = isVertical ? hbar : vbar;
        VirtualScrollBar lengthBar = isVertical ? vbar : hbar;

        final double viewportBreadth = getViewportBreadth();

        final int cellsSize = cells.size();
        final int cellCount = getCellCount();
        for (int i = 0; i < 2; i++) {
            final boolean lengthBarVisible = getPosition() > 0
                    || cellCount > cellsSize
                    || (cellCount == cellsSize && (getCellPosition(cells.getLast()) + getCellLength(cells.getLast())) > getViewportLength())
                    || (cellCount == cellsSize - 1 && barVisibilityChanged && needBreadthBar);

            if (lengthBarVisible ^ needLengthBar) {
                needLengthBar = lengthBarVisible;
                barVisibilityChanged = true;
            }

            // second conditional removed for RT-36669.
            final boolean breadthBarVisible = (maxPrefBreadth > viewportBreadth);// || (needLengthBar && maxPrefBreadth > (viewportBreadth - lengthBarBreadth));
            if (breadthBarVisible ^ needBreadthBar) {
                needBreadthBar = breadthBarVisible;
                barVisibilityChanged = true;
            }
        }

        // Start by optimistically deciding whether the length bar and
        // breadth bar are needed and adjust the viewport dimensions
        // accordingly. If during layout we find that one or the other of the
        // bars actually is needed, then we will perform a cleanup pass

        if (!Properties.IS_TOUCH_SUPPORTED) {
            updateViewportDimensions();
            breadthBar.setVisible(needBreadthBar);
            lengthBar.setVisible(needLengthBar);
        } else {
            breadthBar.setVisible(needBreadthBar && tempVisibility);
            lengthBar.setVisible(needLengthBar && tempVisibility);
        }

        return barVisibilityChanged;
    }

    private void updateViewportDimensions() {
        final boolean isVertical = isVertical();
        final double breadthBarLength = isVertical ? snapSizeY(hbar.prefHeight(-1)) : snapSizeX(vbar.prefWidth(-1));
        final double lengthBarBreadth = isVertical ? snapSizeX(vbar.prefWidth(-1)) : snapSizeY(hbar.prefHeight(-1));

        setViewportBreadth((isVertical ? getWidth() : getHeight()) - (needLengthBar ? lengthBarBreadth : 0));
        setViewportLength((isVertical ? getHeight() : getWidth()) - (needBreadthBar ? breadthBarLength : 0));
    }

    private void initViewport() {
        // Initialize the viewportLength and viewportBreadth to match the
        // width/height of the flow
        final boolean isVertical = isVertical();

        updateViewportDimensions();

        VirtualScrollBar breadthBar = isVertical ? hbar : vbar;
        VirtualScrollBar lengthBar = isVertical ? vbar : hbar;

        // If there has been a switch between the virtualized bar, then we
        // will want to do some stuff TODO.
        breadthBar.setVirtual(false);
        lengthBar.setVirtual(true);
    }

    private void updateScrollBarsAndCells(boolean recreate) {
        // Assign the hbar and vbar to the breadthBar and lengthBar so as
        // to make some subsequent calculations easier.
        final boolean isVertical = isVertical();
        VirtualScrollBar breadthBar = isVertical ? hbar : vbar;
        VirtualScrollBar lengthBar = isVertical ? vbar : hbar;

        // We may have adjusted the viewport length and breadth after the
        // layout due to scroll bars becoming visible. So we need to perform
        // a follow up pass and resize and shift all the cells to fit the
        // viewport. Note that the prospective viewport size is always >= the
        // final viewport size, so we don't have to worry about adding
        // cells during this cleanup phase.
        fitCells();

        // Update cell positions.
        // When rebuilding the cells, we add the cells and along the way compute
        // the maxPrefBreadth. Based on the computed value, we may add
        // the breadth scrollbar which changes viewport length, so we need
        // to re-position the cells.
        if (!cells.isEmpty()) {
            final double currOffset = -computeViewportOffset(getPosition());
            final int currIndex = computeCurrentIndex() - cells.getFirst().getIndex();
            final int size = cells.size();

            // position leading cells
            double offset = currOffset;

            for (int i = currIndex - 1; i >= 0 && i < size; i--) {
                final T cell = cells.get(i);

                offset -= getCellLength(cell);

                positionCell(cell, offset);
            }

            // position trailing cells
            offset = currOffset;
            for (int i = currIndex; i >= 0 && i < size; i++) {
                final T cell = cells.get(i);
                positionCell(cell, offset);

                offset += getCellLength(cell);
            }
        }

        // Toggle visibility on the corner
        corner.setVisible(breadthBar.isVisible() && lengthBar.isVisible());

        double sumCellLength = 0;
        double flowLength = (isVertical ? getHeight() : getWidth()) -
                (breadthBar.isVisible() ? breadthBar.prefHeight(-1) : 0);

        final double viewportBreadth = getViewportBreadth();
        final double viewportLength = getViewportLength();

        // Now position and update the scroll bars
        if (breadthBar.isVisible()) {
            /*
            ** Positioning the ScrollBar
            */
            if (!Properties.IS_TOUCH_SUPPORTED) {
                if (isVertical) {
                    hbar.resizeRelocate(0, viewportLength,
                            viewportBreadth, hbar.prefHeight(viewportBreadth));
                } else {
                    vbar.resizeRelocate(viewportLength, 0,
                            vbar.prefWidth(viewportBreadth), viewportBreadth);
                }
            }
            else {
                if (isVertical) {
                    hbar.resizeRelocate(0, (viewportLength-hbar.getHeight()),
                            viewportBreadth, hbar.prefHeight(viewportBreadth));
                } else {
                    vbar.resizeRelocate((viewportLength-vbar.getWidth()), 0,
                            vbar.prefWidth(viewportBreadth), viewportBreadth);
                }
            }

            if (getMaxPrefBreadth() != -1) {
                double newMax = Math.max(1, getMaxPrefBreadth() - viewportBreadth);
                if (newMax != breadthBar.getMax()) {
                    breadthBar.setMax(newMax);

                    double breadthBarValue = breadthBar.getValue();
                    boolean maxed = breadthBarValue != 0 && newMax == breadthBarValue;
                    if (maxed || breadthBarValue > newMax) {
                        breadthBar.setValue(newMax);
                    }

                    breadthBar.setVisibleAmount((viewportBreadth / getMaxPrefBreadth()) * newMax);
                }
            }
        }

        // determine how many cells there are on screen so that the scrollbar
        // thumb can be appropriately sized
        if (recreate && (lengthBar.isVisible() || Properties.IS_TOUCH_SUPPORTED)) {
            final int cellCount = getCellCount();
            int numCellsVisibleOnScreen = 0;
            for (int i = 0, max = cells.size(); i < max; i++) {
                T cell = cells.get(i);
                if (cell != null && !cell.isEmpty()) {
                    sumCellLength += (isVertical ? cell.getHeight() : cell.getWidth());
                    if (sumCellLength > flowLength) {
                        break;
                    }

                    numCellsVisibleOnScreen++;
                }
            }

            lengthBar.setMax(1);
            if (numCellsVisibleOnScreen == 0 && cellCount == 1) {
                // special case to help resolve RT-17701 and the case where we have
                // only a single row and it is bigger than the viewport
                lengthBar.setVisibleAmount(flowLength / sumCellLength);
            } else {
                lengthBar.setVisibleAmount(numCellsVisibleOnScreen / (float) cellCount);
            }
        }

        if (lengthBar.isVisible()) {
            // Fix for RT-11873. If this isn't here, we can have a situation where
            // the scrollbar scrolls endlessly. This is possible when the cell
            // count grows as the user hits the maximal position on the scrollbar
            // (i.e. the list size dynamically grows as the user needs more).
            //
            // This code was commented out to resolve RT-14477 after testing
            // whether RT-11873 can be recreated. It could not, and therefore
            // for now this code will remained uncommented until it is deleted
            // following further testing.
//            if (lengthBar.getValue() == 1.0 && lastCellCount != cellCount) {
//                lengthBar.setValue(0.99);
//            }

            /*
            ** Positioning the ScrollBar
            */
            if (!Properties.IS_TOUCH_SUPPORTED) {
                if (isVertical) {
                    vbar.resizeRelocate(viewportBreadth, 0, vbar.prefWidth(viewportLength), viewportLength);
                } else {
                    hbar.resizeRelocate(0, viewportBreadth, viewportLength, hbar.prefHeight(-1));
                }
            }
            else {
                if (isVertical) {
                    vbar.resizeRelocate((viewportBreadth-vbar.getWidth()), 0, vbar.prefWidth(viewportLength), viewportLength);
                } else {
                    hbar.resizeRelocate(0, (viewportBreadth-hbar.getHeight()), viewportLength, hbar.prefHeight(-1));
                }
            }
        }

        if (corner.isVisible()) {
            if (!Properties.IS_TOUCH_SUPPORTED) {
                corner.resize(vbar.getWidth(), hbar.getHeight());
                corner.relocate(hbar.getLayoutX() + hbar.getWidth(), vbar.getLayoutY() + vbar.getHeight());
            }
            else {
                corner.resize(vbar.getWidth(), hbar.getHeight());
                corner.relocate(hbar.getLayoutX() + (hbar.getWidth()-vbar.getWidth()), vbar.getLayoutY() + (vbar.getHeight()-hbar.getHeight()));
                hbar.resize(hbar.getWidth()-vbar.getWidth(), hbar.getHeight());
                vbar.resize(vbar.getWidth(), vbar.getHeight()-hbar.getHeight());
            }
        }

        clipView.resize(snapSizeX(isVertical ? viewportBreadth : viewportLength),
                        snapSizeY(isVertical ? viewportLength : viewportBreadth));

        // If the viewportLength becomes large enough that all cells fit
        // within the viewport, then we want to update the value to match.
        if (getPosition() != lengthBar.getValue()) {
            lengthBar.setValue(getPosition());
        }
    }

    
Adjusts the cells location and size if necessary. The breadths of all
cells will be adjusted to fit the viewportWidth or maxPrefBreadth, and
the layout position will be updated if necessary based on index and
offset.
/**
     * Adjusts the cells location and size if necessary. The breadths of all
     * cells will be adjusted to fit the viewportWidth or maxPrefBreadth, and
     * the layout position will be updated if necessary based on index and
     * offset.
     */
    private void fitCells() {
        double size = Math.max(getMaxPrefBreadth(), getViewportBreadth());
        boolean isVertical = isVertical();

        // Note: Do not optimise this loop by pre-calculating the cells size and
        // storing that into a int value - this can lead to RT-32828
        for (int i = 0; i < cells.size(); i++) {
            Cell<?> cell = cells.get(i);
            if (isVertical) {
                cell.resize(size, cell.prefHeight(size));
            } else {
                cell.resize(cell.prefWidth(size), size);
            }
        }
    }

    private void cull() {
        final double viewportLength = getViewportLength();
        for (int i = cells.size() - 1; i >= 0; i--) {
            T cell = cells.get(i);
            double cellSize = getCellLength(cell);
            double cellStart = getCellPosition(cell);
            double cellEnd = cellStart + cellSize;
            if (cellStart >= viewportLength || cellEnd < 0) {
                addToPile(cells.remove(i));
            }
        }
    }

    
After using the accum cell, it needs to be released!
/**
     * After using the accum cell, it needs to be released!
     */
    private void releaseCell(T cell) {
        if (accumCell != null && cell == accumCell) {
            accumCell.updateIndex(-1);
        }
    }

    
This method is an experts-only method - if the requested index is not
already an existing visible cell, it will create a cell for the
given index and insert it into the sheet. From that point on it will be
unmanaged, and is up to the caller of this method to manage it.
/**
     * This method is an experts-only method - if the requested index is not
     * already an existing visible cell, it will create a cell for the
     * given index and insert it into the sheet. From that point on it will be
     * unmanaged, and is up to the caller of this method to manage it.
     */
    T getPrivateCell(int index)  {
        T cell = null;

        // If there are cells, then we will attempt to get an existing cell
        if (! cells.isEmpty()) {
            // First check the cells that have already been created and are
            // in use. If this call returns a value, then we can use it
            cell = getVisibleCell(index);
            if (cell != null) {
                // Force the underlying text inside the cell to be updated
                // so that when the screen reader runs, it will match the
                // text in the cell (force updateDisplayedText())
                cell.layout();
                return cell;
            }
        }

        // check the existing sheet children
        if (cell == null) {
            for (int i = 0; i < sheetChildren.size(); i++) {
                T _cell = (T) sheetChildren.get(i);
                if (getCellIndex(_cell) == index) {
                    return _cell;
                }
            }
        }

        Callback<VirtualFlow<T>, T> cellFactory = getCellFactory();
        if (cellFactory != null) {
            cell = cellFactory.call(this);
        }

        if (cell != null) {
            setCellIndex(cell, index);
            resizeCellSize(cell);
            cell.setVisible(false);
            sheetChildren.add(cell);
            privateCells.add(cell);
        }

        return cell;
    }

    private final List<T> privateCells = new ArrayList<>();

    private void releaseAllPrivateCells() {
        sheetChildren.removeAll(privateCells);
        privateCells.clear();
    }

    
Puts the given cell onto the pile. This is called whenever a cell has
fallen off the flow's start.
/**
     * Puts the given cell onto the pile. This is called whenever a cell has
     * fallen off the flow's start.
     */
    private void addToPile(T cell) {
        assert cell != null;
        pile.addLast(cell);
    }

    private void cleanPile() {
        boolean wasFocusOwner = false;

        for (int i = 0, max = pile.size(); i < max; i++) {
            T cell = pile.get(i);
            wasFocusOwner = wasFocusOwner || doesCellContainFocus(cell);
            cell.setVisible(false);
        }

        // Fix for RT-35876: Rather than have the cells do weird things with
        // focus (in particular, have focus jump between cells), we return focus
        // to the VirtualFlow itself.
        if (wasFocusOwner) {
            requestFocus();
        }
    }

    private boolean doesCellContainFocus(Cell<?> c) {
        Scene scene = c.getScene();
        final Node focusOwner = scene == null ? null : scene.getFocusOwner();

        if (focusOwner != null) {
            if (c.equals(focusOwner)) {
                return true;
            }

            Parent p = focusOwner.getParent();
            while (p != null && ! (p instanceof VirtualFlow)) {
                if (c.equals(p)) {
                    return true;
                }
                p = p.getParent();
            }
        }

        return false;
    }

    private double getPrefBreadth(double oppDimension) {
        double max = getMaxCellWidth(10);

        // This primarily exists for the case where we do not want the breadth
        // to grow to ensure a golden ratio between width and height (for example,
        // when a ListView is used in a ComboBox - the width should not grow
        // just because items are being added to the ListView)
        if (oppDimension > -1) {
            double prefLength = getPrefLength();
            max = Math.max(max, prefLength * GOLDEN_RATIO_MULTIPLIER);
        }

        return max;
    }

    private double getPrefLength() {
        double sum = 0.0;
        int rows = Math.min(10, getCellCount());
        for (int i = 0; i < rows; i++) {
            sum += getCellLength(i);
        }
        return sum;
    }

    double getMaxCellWidth(int rowsToCount) {
        double max = 0.0;

        // we always measure at least one row
        int rows = Math.max(1, rowsToCount == -1 ? getCellCount() : rowsToCount);
        for (int i = 0; i < rows; i++) {
            max = Math.max(max, getCellBreadth(i));
        }
        return max;
    }

    // Old PositionMapper
    
Given a position value between 0 and 1, compute and return the viewport
offset from the "current" cell associated with that position value.
That is, if the return value of this function where used as a translation
factor for a sheet that contained all the items, then the current
item would end up positioned correctly.
/**
     * Given a position value between 0 and 1, compute and return the viewport
     * offset from the "current" cell associated with that position value.
     * That is, if the return value of this function where used as a translation
     * factor for a sheet that contained all the items, then the current
     * item would end up positioned correctly.
     */
    private double computeViewportOffset(double position) {
        double p = com.sun.javafx.util.Utils.clamp(0, position, 1);
        double fractionalPosition = p * getCellCount();
        int cellIndex = (int) fractionalPosition;
        double fraction = fractionalPosition - cellIndex;
        double cellSize = getCellLength(cellIndex);
        double pixelOffset = cellSize * fraction;
        double viewportOffset = getViewportLength() * p;
        return pixelOffset - viewportOffset;
    }

    private void adjustPositionToIndex(int index) {
        int cellCount = getCellCount();
        if (cellCount <= 0) {
            setPosition(0.0f);
        } else {
            setPosition(((double)index) / cellCount);
        }
    }

    
Adjust the position based on a delta of pixels. If negative, then the
position will be adjusted negatively. If positive, then the position will
be adjusted positively. If the pixel amount is too great for the range of
the position, then it will be clamped such that position is always
strictly between 0 and 1
/**
     * Adjust the position based on a delta of pixels. If negative, then the
     * position will be adjusted negatively. If positive, then the position will
     * be adjusted positively. If the pixel amount is too great for the range of
     * the position, then it will be clamped such that position is always
     * strictly between 0 and 1
     */
    private void adjustByPixelAmount(double numPixels) {
        if (numPixels == 0) return;
        // Starting from the current cell, we move in the direction indicated
        // by numPixels one cell at a team. For each cell, we discover how many
        // pixels the "position" line would move within that cell, and adjust
        // our count of numPixels accordingly. When we come to the "final" cell,
        // then we can take the remaining number of pixels and multiply it by
        // the "travel rate" of "p" within that cell to get the delta. Add
        // the delta to "p" to get position.

        // get some basic info about the list and the current cell
        boolean forward = numPixels > 0;
        int cellCount = getCellCount();
        double fractionalPosition = getPosition() * cellCount;
        int cellIndex = (int) fractionalPosition;
        if (forward && cellIndex == cellCount) return;
        double cellSize = getCellLength(cellIndex);
        double fraction = fractionalPosition - cellIndex;
        double pixelOffset = cellSize * fraction;

        // compute the percentage of "position" that represents each cell
        double cellPercent = 1.0 / cellCount;

        // To help simplify the algorithm, we pretend as though the current
        // position is at the beginning of the current cell. This reduces some
        // of the corner cases and provides a simpler algorithm without adding
        // any overhead to performance.
        double start = computeOffsetForCell(cellIndex);
        double end = cellSize + computeOffsetForCell(cellIndex + 1);

        // We need to discover the distance that the fictional "position line"
        // would travel within this cell, from its current position to the end.
        double remaining = end - start;

        // Keep track of the number of pixels left to travel
        double n = forward ?
              numPixels + pixelOffset - (getViewportLength() * getPosition()) - start
            : -numPixels + end - (pixelOffset - (getViewportLength() * getPosition()));

        // "p" represents the most recent value for position. This is always
        // based on the edge between two cells, except at the very end of the
        // algorithm where it is added to the computed "p" offset for the final
        // value of Position.
        double p = cellPercent * cellIndex;

        // Loop over the cells one at a time until either we reach the end of
        // the cells, or we find that the "n" will fall within the cell we're on
        while (n > remaining && ((forward && cellIndex < cellCount - 1) || (! forward && cellIndex > 0))) {
            if (forward) cellIndex++; else cellIndex--;
            n -= remaining;
            cellSize = getCellLength(cellIndex);
            start = computeOffsetForCell(cellIndex);
            end = cellSize + computeOffsetForCell(cellIndex + 1);
            remaining = end - start;
            p = cellPercent * cellIndex;
        }

        // if remaining is < n, then we must have hit an end, so as a
        // fast path, we can just set position to 1.0 or 0.0 and return
        // because we know we hit the end
        if (n > remaining) {
            setPosition(forward ? 1.0f : 0.0f);
        } else if (forward) {
            double rate = cellPercent / Math.abs(end - start);
            setPosition(p + (rate * n));
        } else {
            double rate = cellPercent / Math.abs(end - start);
            setPosition((p + cellPercent) - (rate * n));
        }
    }

    private int computeCurrentIndex() {
        return (int) (getPosition() * getCellCount());
    }

    
Given an item index, this function will compute and return the viewport
offset from the beginning of the specified item. Notice that because each
item has the same percentage of the position dedicated to it, and since
we are measuring from the start of each item, this is a very simple
calculation.
/**
     * Given an item index, this function will compute and return the viewport
     * offset from the beginning of the specified item. Notice that because each
     * item has the same percentage of the position dedicated to it, and since
     * we are measuring from the start of each item, this is a very simple
     * calculation.
     */
    private double computeOffsetForCell(int itemIndex) {
        double cellCount = getCellCount();
        double p = com.sun.javafx.util.Utils.clamp(0, itemIndex, cellCount) / cellCount;
        return -(getViewportLength() * p);
    }

//    /**
//     * Adjust the position based on a chunk of pixels. The position is based
//     * on the start of the scrollbar position.
//     */
//    private void adjustByPixelChunk(double numPixels) {
//        setPosition(0);
//        adjustByPixelAmount(numPixels);
//    }
    // end of old PositionMapper code




    /***************************************************************************
     *                                                                         *
     * Support classes                                                         *
     *                                                                         *
     **************************************************************************/

    
A simple extension to Region that ensures that anything wanting to flow
outside of the bounds of the Region is clipped.
/**
     * A simple extension to Region that ensures that anything wanting to flow
     * outside of the bounds of the Region is clipped.
     */
    static class ClippedContainer extends Region {

        
The Node which is embedded within this ClipView. /**
         * The Node which is embedded within this {@code ClipView}.
         */
        private Node node;
        public Node getNode() { return this.node; }
        public void setNode(Node n) {
            this.node = n;

            getChildren().clear();
            getChildren().add(node);
        }

        public void setClipX(double clipX) {
            setLayoutX(-clipX);
            clipRect.setLayoutX(clipX);
        }

        public void setClipY(double clipY) {
            setLayoutY(-clipY);
            clipRect.setLayoutY(clipY);
        }

        private final Rectangle clipRect;

        public ClippedContainer(final VirtualFlow<?> flow) {
            if (flow == null) {
                throw new IllegalArgumentException("VirtualFlow can not be null");
            }

            getStyleClass().add("clipped-container");

            // clipping
            clipRect = new Rectangle();
            clipRect.setSmooth(false);
            setClip(clipRect);
            // --- clipping

            super.widthProperty().addListener(valueModel -> {
                clipRect.setWidth(getWidth());
            });
            super.heightProperty().addListener(valueModel -> {
                clipRect.setHeight(getHeight());
            });
        }
    }

    
A List-like implementation that is exceedingly efficient for the purposes
of the VirtualFlow. Typically there is not much variance in the number of
cells -- it is always some reasonably consistent number. Yet for efficiency
in code, we like to use a linked list implementation so as to append to
start or append to end. However, at times when we need to iterate, LinkedList
is expensive computationally as well as requiring the construction of
temporary iterators.

This linked list like implementation is done using an array. It begins by
putting the first item in the center of the allocated array, and then grows
outward (either towards the first or last of the array depending on whether
we are inserting at the head or tail). It maintains an index to the start
and end of the array, so that it can efficiently expose iteration.

This class is package private solely for the sake of testing.
/**
     * A List-like implementation that is exceedingly efficient for the purposes
     * of the VirtualFlow. Typically there is not much variance in the number of
     * cells -- it is always some reasonably consistent number. Yet for efficiency
     * in code, we like to use a linked list implementation so as to append to
     * start or append to end. However, at times when we need to iterate, LinkedList
     * is expensive computationally as well as requiring the construction of
     * temporary iterators.
     * <p>
     * This linked list like implementation is done using an array. It begins by
     * putting the first item in the center of the allocated array, and then grows
     * outward (either towards the first or last of the array depending on whether
     * we are inserting at the head or tail). It maintains an index to the start
     * and end of the array, so that it can efficiently expose iteration.
     * <p>
     * This class is package private solely for the sake of testing.
     */
    static class ArrayLinkedList<T> extends AbstractList<T> {
        
The array list backing this class. We default the size of the array
list to be fairly large so as not to require resizing during normal
use, and since that many ArrayLinkedLists won't be created it isn't
very painful to do so.
/**
         * The array list backing this class. We default the size of the array
         * list to be fairly large so as not to require resizing during normal
         * use, and since that many ArrayLinkedLists won't be created it isn't
         * very painful to do so.
         */
        private final ArrayList<T> array;

        private int firstIndex = -1;
        private int lastIndex = -1;

        public ArrayLinkedList() {
            array = new ArrayList<T>(50);

            for (int i = 0; i < 50; i++) {
                array.add(null);
            }
        }

        public T getFirst() {
            return firstIndex == -1 ? null : array.get(firstIndex);
        }

        public T getLast() {
            return lastIndex == -1 ? null : array.get(lastIndex);
        }

        public void addFirst(T cell) {
            // if firstIndex == -1 then that means this is the first item in the
            // list and we need to initialize firstIndex and lastIndex
            if (firstIndex == -1) {
                firstIndex = lastIndex = array.size() / 2;
                array.set(firstIndex, cell);
            } else if (firstIndex == 0) {
                // we're already at the head of the array, so insert at position
                // 0 and then increment the lastIndex to compensate
                array.add(0, cell);
                lastIndex++;
            } else {
                // we're not yet at the head of the array, so insert at the
                // firstIndex - 1 position and decrement first position
                array.set(--firstIndex, cell);
            }
        }

        public void addLast(T cell) {
            // if lastIndex == -1 then that means this is the first item in the
            // list and we need to initialize the firstIndex and lastIndex
            if (firstIndex == -1) {
                firstIndex = lastIndex = array.size() / 2;
                array.set(lastIndex, cell);
            } else if (lastIndex == array.size() - 1) {
                // we're at the end of the array so need to "add" so as to force
                // the array to be expanded in size
                array.add(++lastIndex, cell);
            } else {
                array.set(++lastIndex, cell);
            }
        }

        public int size() {
            return firstIndex == -1 ? 0 : lastIndex - firstIndex + 1;
        }

        public boolean isEmpty() {
            return firstIndex == -1;
        }

        public T get(int index) {
            if (index > (lastIndex - firstIndex) || index < 0) {
                // Commented out exception due to RT-29111
                // throw new java.lang.ArrayIndexOutOfBoundsException();
                return null;
            }

            return array.get(firstIndex + index);
        }

        public void clear() {
            for (int i = 0; i < array.size(); i++) {
                array.set(i, null);
            }

            firstIndex = lastIndex = -1;
        }

        public T removeFirst() {
            if (isEmpty()) return null;
            return remove(0);
        }

        public T removeLast() {
            if (isEmpty()) return null;
            return remove(lastIndex - firstIndex);
        }

        public T remove(int index) {
            if (index > lastIndex - firstIndex || index < 0) {
                throw new ArrayIndexOutOfBoundsException();
            }

            // if the index == 0, then we're removing the first
            // item and can simply set it to null in the array and increment
            // the firstIndex unless there is only one item, in which case
            // we have to also set first & last index to -1.
            if (index == 0) {
                T cell = array.get(firstIndex);
                array.set(firstIndex, null);
                if (firstIndex == lastIndex) {
                    firstIndex = lastIndex = -1;
                } else {
                    firstIndex++;
                }
                return cell;
            } else if (index == lastIndex - firstIndex) {
                // if the index == lastIndex - firstIndex, then we're removing the
                // last item and can simply set it to null in the array and
                // decrement the lastIndex
                T cell = array.get(lastIndex);
                array.set(lastIndex--, null);
                return cell;
            } else {
                // if the index is somewhere in between, then we have to remove the
                // item and decrement the lastIndex
                T cell = array.get(firstIndex + index);
                array.set(firstIndex + index, null);
                for (int i = (firstIndex + index + 1); i <= lastIndex; i++) {
                    array.set(i - 1, array.get(i));
                }
                array.set(lastIndex--, null);
                return cell;
            }
        }
    }
}