Workaround to correct trackpad scrolling gestures under macOS

src/main/java/org/fxmisc/flowless/CellListManager.java

@@ -5,6 +5,7 @@
 
 import javafx.collections.ObservableList;
 import javafx.scene.Node;
+import javafx.scene.input.ScrollEvent;
 
 import org.reactfx.EventStreams;
 import org.reactfx.Subscription;
@@ -18,6 +19,7 @@
  */
 final class CellListManager<T, C extends Cell<T, ? extends Node>> {
 
+    private final Node owner;
     private final CellPool<T, C> cellPool;
     private final MemoizationList<C> cells;
     private final LiveList<C> presentCells;
@@ -26,8 +28,10 @@ final class CellListManager<T, C extends Cell<T, ? extends Node>> {
     private final Subscription presentCellsSubscription;
 
     public CellListManager(
+            Node owner,
             ObservableList<T> items,
             Function<? super T, ? extends C> cellFactory) {
+        this.owner = owner;
         this.cellPool = new CellPool<>(cellFactory);
         this.cells = LiveList.map(items, this::cellForItem).memoize();
         this.presentCells = cells.memoizedItems();
@@ -96,9 +100,38 @@ private C cellForItem(T item) {
         // It will be made visible when it is positioned.
         node.setVisible(false);
 
+        if (cell.isReusable()) {
+            // if cell is reused i think adding event handler
+            // would cause resource leakage.
+            node.setOnScroll(this::pushScrollEvent);
+            node.setOnScrollStarted(this::pushScrollEvent);
+            node.setOnScrollFinished(this::pushScrollEvent);
+        } else {
+            node.addEventHandler(ScrollEvent.ANY, this::pushScrollEvent);
+        }
+
         return cell;
     }
 
+    /**
+     * Push scroll events received by cell nodes directly to
+     * the 'owner' Node. (Generally likely to be a VirtualFlow
+     * but not required.)
+     *
+     * Normal bubbling of scroll events gets interrupted during
+     * a scroll gesture when the Cell's Node receiving the event
+     * has moved out of the viewport and is thus removed from
+     * the Navigator's children list. This breaks expected trackpad
+     * scrolling behaviour, at least on macOS.
+     * 
+     * So here we take over event-bubbling duties for ScrollEvent
+     * and push them ourselves directly to the given owner.
+     */
+    private void pushScrollEvent(ScrollEvent se) {
+        owner.fireEvent(se);
+        se.consume();
+    }
+
     private void presentCellsChanged(QuasiListModification<? extends C> mod) {
         // add removed cells back to the pool
         for(C cell: mod.getRemoved()) {

src/main/java/org/fxmisc/flowless/VirtualFlow.java

@@ -168,7 +168,7 @@ private VirtualFlow(
         this.getStyleClass().add("virtual-flow");
         this.items = items;
         this.orientation = orientation;
-        this.cellListManager = new CellListManager<>(items, cellFactory);
+        this.cellListManager = new CellListManager<>(this, items, cellFactory);
         this.gravity.set(gravity);
         MemoizationList<C> cells = cellListManager.getLazyCellList();
         this.sizeTracker = new SizeTracker(orientation, layoutBoundsProperty(), cells);
@@ -183,7 +183,7 @@ private VirtualFlow(
         lengthOffsetEstimate = sizeTracker.lengthOffsetEstimateProperty().asVar(this::setLengthOffset);
 
         // scroll content by mouse scroll
-        this.addEventHandler(ScrollEvent.SCROLL, se -> {
+        this.addEventHandler(ScrollEvent.ANY, se -> {
             scrollXBy(-se.getDeltaX());
             scrollYBy(-se.getDeltaY());
             se.consume();
@@ -634,4 +634,4 @@ public StyleableProperty<Gravity> getStyleableProperty(VirtualFlow virtualFlow)
     public List<CssMetaData<? extends Styleable, ?>> getCssMetaData() {
         return getClassCssMetaData();
     }
-}
+}