8291908: VirtualFlow creates unneeded empty cells

Reviewed-by: kcr, aghaisas

Author: Johan Vos

Diff for: modules/javafx.controls/src/main/java/javafx/scene/control/skin/VirtualFlow.java

@@ -2072,7 +2072,7 @@ protected T getFirstVisibleCellWithinViewport() {
      */
     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
+        // viewport to the top of the current index. We will decrement it
         // as we lay out leading cells.
         double offset = startOffset;
         // The index is the absolute index of the cell being laid out
@@ -2877,6 +2877,10 @@ private double computeViewportOffset(double position) {
         double p = com.sun.javafx.util.Utils.clamp(0, position, 1);
         double bound = 0d;
         double estSize = estimatedSize / getCellCount();
+        double maxOff = estimatedSize - getViewportLength();
+        if ((maxOff > 0) && (absoluteOffset > maxOff)) {
+            return maxOff - absoluteOffset;
+        }
 
         for (int i = 0; i < getCellCount(); i++) {
             double h = getCellSize(i);

Diff for: modules/javafx.controls/src/test/java/test/javafx/scene/control/TreeTableViewTest.java

@@ -3669,7 +3669,7 @@ protected <T> void assert_rt35857(ObservableList<T> items, MultipleSelectionMode
         // However, for now, we'll test on the assumption that across all
         // platforms we only get one extra cell created, and we can loosen this
         // up if necessary.
-        assertEquals(cellCountAtStart + 14, rt36452_instanceCount);
+        assertTrue(rt36452_instanceCount < cellCountAtStart + 15);
 
         sl.dispose();
     }

Diff for: modules/javafx.controls/src/test/java/test/javafx/scene/control/skin/VirtualFlowTest.java

@@ -1254,6 +1254,40 @@ public void testSheetChildrenRemainsConstant() {
         assertEquals("Wrong number of sheet children after removing all items", 12, sheetChildrenSize);
     }
 
+    @Test
+    // See JDK-8291908
+    public void test_noEmptyTrailingCells() {
+        flow = new VirtualFlowShim();
+        flow.setVertical(true);
+        flow.setCellFactory(p -> new CellStub(flow) {
+            @Override
+            protected double computeMaxHeight(double width) {
+                return computePrefHeight(width);
+            }
+
+            @Override
+            protected double computePrefHeight(double width) {
+                return (getIndex() > 100) ? 1 : 20;
+            }
+
+            @Override
+            protected double computeMinHeight(double width) {
+                return computePrefHeight(width);
+            }
+
+        });
+        flow.setCellCount(100);
+        flow.setViewportLength(1000);
+        flow.resize(100, 1000);
+        pulse();
+        flow.sheetChildren.addListener((InvalidationListener) (o) -> {
+            int count = ((List) o).size();
+            assertTrue(Integer.toString(count), count < 101);
+        });
+        flow.scrollTo(99);
+        pulse();
+    }
+
     private ArrayLinkedListShim<GraphicalCellStub> circlelist = new ArrayLinkedListShim<GraphicalCellStub>();
 
     private VirtualFlowShim createCircleFlow() {