Fix: Flicker when reorderable list doesn't change its position (flutter#151026)

Fix: Flicker when no update in index of dragged item

Resolves flutter#150843

This PR ensures that even if we move dragged item anywhere in list and comeback to initial position, we smoothly adjust to that position.

Author: rkishan516

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

@@ -871,7 +871,7 @@ class SliverReorderableListState extends State<SliverReorderableList> with Ticke
     // Find the new index for inserting the item being dragged.
     int newIndex = _insertIndex!;
     for (final _ReorderableItemState item in _items.values) {
-      if (item.index == _dragIndex! || !item.mounted) {
+      if ((_reverse && item.index == _dragIndex!) || !item.mounted) {
         continue;
       }
 
@@ -902,7 +902,13 @@ class SliverReorderableListState extends State<SliverReorderableList> with Ticke
           newIndex = item.index;
         }
       } else {
-        if (itemStart <= proxyItemStart && proxyItemStart <= itemMiddle) {
+        if (item.index == _dragIndex!) {
+          // If end of the proxy is not in ending half of item,
+          // we don't process, because it's original dragged item.
+          if (itemMiddle <= proxyItemEnd && proxyItemEnd <= itemEnd) {
+            newIndex = _dragIndex!;
+          }
+        } else if (itemStart <= proxyItemStart && proxyItemStart <= itemMiddle) {
           // The start of the proxy is in the beginning half of the item, so
           // we should swap the item with the gap and we are done looking for
           // the new index.

packages/flutter/test/widgets/reorderable_list_test.dart

@@ -717,6 +717,68 @@ void main() {
     expect(tester.getTopLeft(find.text('item 0')), const Offset(0, 500));
   });
 
+  testWidgets('SliverReorderableList - properly animates the drop at starting position', (WidgetTester tester) async {
+    // Regression test for https://github.com/flutter/flutter/issues/150843
+    final List<int> items = List<int>.generate(3, (int index) => index);
+
+    // The TestList is 300x100 SliverReorderableList with 3 items 100x100 each.
+    // Each item has a text widget with 'item $index' that can be moved by a
+    // press and drag gesture. For this test the first item is at the top
+    await tester.pumpWidget(TestList(items: items));
+
+    expect(tester.getTopLeft(find.text('item 0')), Offset.zero);
+    expect(tester.getTopLeft(find.text('item 1')), const Offset(0, 100));
+
+    // Drag item 0 downwards and then upwards.
+    final TestGesture drag = await tester.startGesture(tester.getCenter(find.text('item 0')));
+    await tester.pump(kPressTimeout);
+    await drag.moveBy(const Offset(0, 100));
+    await tester.pumpAndSettle();
+    await drag.moveBy(const Offset(0, -110));
+    await tester.pump();
+    expect(tester.getTopLeft(find.text('item 0')), const Offset(0, -10));
+    expect(tester.getTopLeft(find.text('item 1')), const Offset(0, 100));
+
+    // Now leave the drag, it should go to index 0.
+    await drag.up();
+    await tester.pump();
+
+    // It should not go to index 1 and come back
+    await tester.pump(const Duration(milliseconds: 200));
+    expect(tester.getTopLeft(find.text('item 0')).dy, lessThan(50));
+  });
+
+  testWidgets('SliverReorderableList - properly animates the drop at starting position with reverse:true', (WidgetTester tester) async {
+    // Regression test for https://github.com/flutter/flutter/issues/150843
+    final List<int> items = List<int>.generate(3, (int index) => index);
+
+    // The TestList is 300x100 SliverReorderableList with 3 items 100x100 each.
+    // Each item has a text widget with 'item $index' that can be moved by a
+    // press and drag gesture. For this test the first item is at the top
+    await tester.pumpWidget(TestList(items: items,reverse: true));
+
+    expect(tester.getTopLeft(find.text('item 2')), const Offset(0, 300.0));
+    expect(tester.getTopLeft(find.text('item 1')), const Offset(0, 400.0));
+
+    // Drag item 2 downwards and then upwards.
+    final TestGesture drag = await tester.startGesture(tester.getCenter(find.text('item 2')));
+    await tester.pump(kPressTimeout);
+    await drag.moveBy(const Offset(0, 100));
+    await tester.pumpAndSettle();
+    await drag.moveBy(const Offset(0, -110));
+    await tester.pump();
+    expect(tester.getTopLeft(find.text('item 2')), const Offset(0, 290));
+    expect(tester.getTopLeft(find.text('item 1')), const Offset(0, 300));
+
+    // Now leave the drag, it should go to index 1.
+    await drag.up();
+    await tester.pump();
+
+    // It should not go to index 0 and come back
+    await tester.pump(const Duration(milliseconds: 200));
+    expect(tester.getTopLeft(find.text('item 2')).dy, greaterThan(350));
+  });
+
   testWidgets('SliverReorderableList calls onReorderStart and onReorderEnd correctly', (WidgetTester tester) async {
     final List<int> items = List<int>.generate(8, (int index) => index);
     int? startIndex, endIndex;