Fix an reorderable list animation issue:"Reversed ReorderableListView drop animation moves item one row higher than it should #110949" (#111027)

Author: hannah-hyj

packages/flutter/lib/src/widgets/reorderable_list.dart

@@ -758,9 +758,11 @@ class SliverReorderableListState extends State<SliverReorderableList> with Ticke
 
   void _dragEnd(_DragInfo item) {
     setState(() {
-      if (_insertIndex! < widget.itemCount - 1) {
-        // Find the location of the item we want to insert before
+      if (_insertIndex == item.index) {
         _finalDropPosition = _itemOffsetAt(_insertIndex! + (_reverse ? 1 : 0));
+      } else if (_insertIndex! < widget.itemCount - 1) {
+        // Find the location of the item we want to insert before
+        _finalDropPosition = _itemOffsetAt(_insertIndex!);
       } else {
         // Inserting into the last spot on the list. If it's the only spot, put
         // it back where it was. Otherwise, grab the second to last and move

packages/flutter/test/widgets/reorderable_list_test.dart

@@ -521,6 +521,57 @@ void main() {
     expect(tester.takeException(), isNull);
   });
 
+  testWidgets('SliverReorderableList - properly animates the drop in a reversed list', (WidgetTester tester) async {
+    // Regression test for https://github.com/flutter/flutter/issues/110949
+    final List<int> items = List<int>.generate(8, (int index) => index);
+
+    Future<void> pressDragRelease(Offset start, Offset delta) async {
+      final TestGesture drag = await tester.startGesture(start);
+      await tester.pump(kPressTimeout);
+      await drag.moveBy(delta);
+      await tester.pumpAndSettle();
+      await drag.up();
+      await tester.pump();
+    }
+
+    // The TestList is 800x600 SliverReorderableList with 8 items 800x100 each.
+    // Each item has a text widget with 'item $index' that can be moved by a
+    // press and drag gesture. For this test we are reversing the order so
+    // the first item is at the bottom.
+    await tester.pumpWidget(TestList(items: items, reverse: true));
+
+    expect(tester.getTopLeft(find.text('item 0')), const Offset(0, 500));
+    expect(tester.getTopLeft(find.text('item 2')), const Offset(0, 300));
+
+    // Drag item 0 up and insert it between item 1 and item 2. It should
+    // smoothly animate.
+    await pressDragRelease(tester.getCenter(find.text('item 0')), const Offset(0, -50));
+    expect(tester.getTopLeft(find.text('item 0')), const Offset(0, 450));
+    expect(tester.getTopLeft(find.text('item 1')), const Offset(0, 500));
+    expect(tester.getTopLeft(find.text('item 2')), const Offset(0, 300));
+
+    // After the first several frames we should be moving closer to the final position,
+    // not further away as was the case with the original bug.
+    await tester.pump(const Duration(milliseconds: 10));
+    expect(tester.getTopLeft(find.text('item 0')).dy, lessThan(450));
+    expect(tester.getTopLeft(find.text('item 0')).dy, greaterThan(400));
+
+    // Sample the middle (don't use exact values as it depends on the internal
+    // curve being used).
+    await tester.pump(const Duration(milliseconds: 125));
+    expect(tester.getTopLeft(find.text('item 0')).dy, lessThan(450));
+    expect(tester.getTopLeft(find.text('item 0')).dy, greaterThan(400));
+
+    // Sample the end of the animation.
+    await tester.pump(const Duration(milliseconds: 100));
+    expect(tester.getTopLeft(find.text('item 0')).dy, lessThan(450));
+    expect(tester.getTopLeft(find.text('item 0')).dy, greaterThan(400));
+
+    // Wait for it to finish, it should be back to the original position
+    await tester.pumpAndSettle();
+    expect(tester.getTopLeft(find.text('item 0')), const Offset(0, 400));
+  });
+
   testWidgets('SliverReorderableList - properly animates the drop at starting position in a reversed list', (WidgetTester tester) async {
     // Regression test for https://github.com/flutter/flutter/issues/84625
     final List<int> items = List<int>.generate(8, (int index) => index);