fix reorderable_list drop animation (#139362)

This PR fixes reorderable_list drop animation.

Fixes #138994

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

@@ -802,40 +802,27 @@ class SliverReorderableListState extends State<SliverReorderableList> with Ticke
   }
 
   void _dragEnd(_DragInfo item) {
-    // No changes required if last child is being inserted into the last position.
-    if ((_insertIndex! + 1 == _items.length) && _reverse) {
-      final RenderBox lastItemRenderBox =  _items[_items.length - 1]!.context.findRenderObject()! as RenderBox;
-      final Offset lastItemOffset =  lastItemRenderBox.localToGlobal(Offset.zero);
-
-      // When drag starts, the corresponding element is removed from
-      // the list, and moves inside of [ReorderableListState.CustomScrollView],
-      // which gives [CustomScrollView] a variable height.
-      //
-      // So when the element is moved, delta would change accordingly,
-      // and since it's the last element,
-      // we animate it back to it's position and add it back to the list.
-      final double delta = item.itemSize.height;
-
-      setState(() {
-        _finalDropPosition = Offset(lastItemOffset.dx, lastItemOffset.dy  - delta);
-      });
-      return;
-    }
     setState(() {
       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!);
+        // Although it's at its original position, the original position has been replaced by a zero-size box
+        // So when reversed, it should offset its own extent
+        _finalDropPosition = _reverse ? _itemOffsetAt(_insertIndex!) - _extentOffset(item.itemExtent, _scrollDirection) : _itemOffsetAt(_insertIndex!);
+      } else if (_reverse) {
+        if (_insertIndex! >= _items.length) {
+          // Drop at the starting position of the last element and offset its own extent
+          _finalDropPosition = _itemOffsetAt(_items.length - 1) - _extentOffset(item.itemExtent, _scrollDirection);
         } 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
-        // down by the gap.
-        final int itemIndex = _items.length > 1 ? _insertIndex! - 1 : _insertIndex!;
-        if (_reverse) {
-          _finalDropPosition = _itemOffsetAt(itemIndex) - _extentOffset(item.itemExtent, _scrollDirection);
+          // Drop at the end of the current element occupying the insert position
+          _finalDropPosition = _itemOffsetAt(_insertIndex!) + _extentOffset(_itemExtentAt(_insertIndex!), _scrollDirection);
+        }
+      } else {
+        if (_insertIndex! == 0) {
+          // Drop at the starting position of the first element and offset its own extent
+          _finalDropPosition = _itemOffsetAt(0) - _extentOffset(item.itemExtent, _scrollDirection);
         } else {
-          _finalDropPosition = _itemOffsetAt(itemIndex) + _extentOffset(item.itemExtent, _scrollDirection);
+          // Drop at the end of the previous element occupying the insert position
+          final int atIndex = _insertIndex! - 1;
+          _finalDropPosition = _itemOffsetAt(atIndex) + _extentOffset(_itemExtentAt(atIndex), _scrollDirection);
         }
       }
     });
@@ -975,8 +962,11 @@ class SliverReorderableListState extends State<SliverReorderableList> with Ticke
   }
 
   Offset _itemOffsetAt(int index) {
-    final RenderBox itemRenderBox =  _items[index]!.context.findRenderObject()! as RenderBox;
-    return itemRenderBox.localToGlobal(Offset.zero);
+    return _items[index]!.targetGeometry().topLeft;
+  }
+
+  double _itemExtentAt(int index) {
+    return _sizeExtent(_items[index]!.targetGeometry().size, _scrollDirection);
   }
 
   Widget _itemBuilder(BuildContext context, int index) {

packages/flutter/test/widgets/reorderable_list_test.dart

@@ -2,6 +2,7 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+import 'package:collection/collection.dart';
 import 'package:flutter/gestures.dart';
 import 'package:flutter/material.dart';
 import 'package:flutter/rendering.dart';
@@ -1405,6 +1406,84 @@ void main() {
       await tester.pumpAndSettle();
     },
   );
+
+  testWidgetsWithLeakTracking('Tests the correctness of the drop animation in various scenarios', (WidgetTester tester) async {
+    // Regression test for https://github.com/flutter/flutter/issues/138994
+    late Size screenSize;
+    final List<double> itemSizes = <double>[20, 50, 30, 80, 100, 30];
+    Future<void> pumpFor(bool reverse, Axis scrollDirection) async {
+      await tester.pumpWidget(
+        MaterialApp(
+          home: Builder(
+            builder: (BuildContext context) {
+              screenSize = MediaQuery.sizeOf(context);
+              return Scaffold(
+                body: CustomScrollView(
+                  reverse: reverse,
+                  scrollDirection: scrollDirection,
+                  slivers: <Widget>[
+                    SliverReorderableList(
+                      itemBuilder: (BuildContext context, int index) {
+                        return ReorderableDragStartListener(
+                          key: ValueKey<int>(index),
+                          index: index,
+                          child: Builder(
+                            builder: (BuildContext context) {
+                              return SizedBox(
+                                height: scrollDirection == Axis.vertical ? itemSizes[index] : double.infinity,
+                                width: scrollDirection == Axis.horizontal ? itemSizes[index] : double.infinity,
+                                child: Text('$index'),
+                              );
+                            },
+                          ),
+                        );
+                      },
+                      itemCount: itemSizes.length,
+                      onReorder: (int fromIndex, int toIndex) {},
+                    ),
+                  ],
+                ),
+              );
+            }
+          ),
+        ),
+      );
+    }
+
+    Future<void> testMove(int from, int to, {bool reverse = false, Axis scrollDirection = Axis.vertical}) async {
+      await pumpFor(reverse, scrollDirection);
+      final double targetOffset = (List<double>.of(itemSizes)..removeAt(from)).sublist(0, to).sum;
+      final Offset targetPosition = reverse
+        ? (scrollDirection == Axis.vertical
+            ? Offset(0, screenSize.height - targetOffset - itemSizes[from])
+            : Offset(screenSize.width - targetOffset - itemSizes[from], 0))
+        : (scrollDirection == Axis.vertical ? Offset(0, targetOffset) : Offset(targetOffset, 0));
+      final Offset moveOffset = targetPosition - tester.getTopLeft(find.text('$from'));
+      await tester.timedDrag(find.text('$from'), moveOffset, const Duration(seconds: 1));
+      // Before the drop animation starts
+      final Offset animationBeginOffset = tester.getTopLeft(find.text('$from'));
+      // Halfway through the animation
+      await tester.pump(const Duration(milliseconds: 125));
+      expect(tester.getTopLeft(find.text('$from')), Offset.lerp(animationBeginOffset, targetPosition, 0.5));
+      // Animation ends
+      await tester.pump(const Duration(milliseconds: 125));
+      expect(tester.getTopLeft(find.text('$from')), targetPosition);
+      await tester.pumpAndSettle();
+    }
+    final List<(int,int)> testCases = <(int,int)>[
+      (3, 1),
+      (3, 3),
+      (3, 5),
+      (0, 5),
+      (5, 0),
+    ];
+    for (final (int, int) element in testCases) {
+      await testMove(element.$1, element.$2);
+      await testMove(element.$1, element.$2, reverse: true);
+      await testMove(element.$1, element.$2, scrollDirection: Axis.horizontal);
+      await testMove(element.$1, element.$2, reverse: true, scrollDirection: Axis.horizontal);
+    }
+  });
 }
 
 class TestList extends StatelessWidget {