Fixed a problem with the first drag update on a reorderable list. (#82296)

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

@@ -6,6 +6,7 @@ import 'dart:math';
 
 import 'package:flutter/gestures.dart';
 import 'package:flutter/rendering.dart';
+import 'package:flutter/scheduler.dart';
 
 import 'basic.dart';
 import 'debug.dart';
@@ -521,6 +522,15 @@ class SliverReorderableListState extends State<SliverReorderableList> with Ticke
   Offset? _finalDropPosition;
   MultiDragGestureRecognizer? _recognizer;
   bool _autoScrolling = false;
+  // To implement the gap for the dragged item, we replace the dragged item
+  // with a zero sized box, and then translate all of the later items down
+  // by the size of the dragged item. This allows us to keep the order of the
+  // list, while still being able to animate the gap between the items. However
+  // for the first frame of the drag, the item has not yet been replaced, so
+  // the calculation for the gap is off by the size of the gap. This flag is
+  // used to determine if the transition to the zero sized box has completed,
+  // so the gap calculation can compensate for it.
+  bool _dragStartTransitionComplete = false;
 
   late ScrollableState _scrollable;
   Axis get _scrollDirection => axisDirectionToAxis(_scrollable.axisDirection);
@@ -615,6 +625,10 @@ class SliverReorderableListState extends State<SliverReorderableList> with Ticke
     final _ReorderableItemState item = _items[_dragIndex!]!;
     item.dragging = true;
     item.rebuild();
+    _dragStartTransitionComplete = false;
+    SchedulerBinding.instance!.addPostFrameCallback((Duration duration) {
+      _dragStartTransitionComplete = true;
+    });
 
     _insertIndex = item.index;
     _dragInfo = _DragInfo(
@@ -722,7 +736,14 @@ class SliverReorderableListState extends State<SliverReorderableList> with Ticke
       if (item.index == _dragIndex! || !item.mounted)
         continue;
 
-      final Rect geometry = item.targetGeometry();
+      Rect geometry = item.targetGeometry();
+      if (!_dragStartTransitionComplete && _dragIndex! <= item.index) {
+        // Transition is not complete, so each item after the dragged item is still
+        // in its normal location and not moved up for the zero sized box that will
+        // replace the dragged item.
+        final Offset transitionOffset = _extentOffset(_reverse ? -gapExtent : gapExtent, _scrollDirection);
+        geometry = (geometry.topLeft - transitionOffset) & geometry.size;
+      }
       final double itemStart = _scrollDirection == Axis.vertical ? geometry.top : geometry.left;
       final double itemExtent = _scrollDirection == Axis.vertical ? geometry.height : geometry.width;
       final double itemEnd = itemStart + itemExtent;

packages/flutter/test/widgets/reorderable_list_test.dart

@@ -122,7 +122,7 @@ void main() {
     expect(items, orderedEquals(<int>[0, 1, 2, 3, 4, 5, 6, 7]));
 
     // Drag item 0 downwards more than halfway to displace item 1.
-    await pressDragRelease(tester.getCenter(find.text('item 0')), const Offset(0, 151));
+    await pressDragRelease(tester.getCenter(find.text('item 0')), const Offset(0, 51));
     check(visible: <int>[0, 1, 2, 3, 4, 5], hidden: <int>[6, 7]);
     expect(tester.getTopLeft(find.text('item 1')), Offset.zero);
     expect(tester.getTopLeft(find.text('item 0')), const Offset(0, 100));
@@ -136,7 +136,7 @@ void main() {
     expect(items, orderedEquals(<int>[0, 1, 2, 3, 4, 5, 6, 7]));
 
     // Drag item 1 to item 3
-    await pressDragRelease(tester.getCenter(find.text('item 1')), const Offset(0, 251));
+    await pressDragRelease(tester.getCenter(find.text('item 1')), const Offset(0, 151));
     check(visible: <int>[0, 1, 2, 3, 4, 5], hidden: <int>[6, 7]);
     expect(tester.getTopLeft(find.text('item 0')), Offset.zero);
     expect(tester.getTopLeft(find.text('item 1')), const Offset(0, 300));
@@ -433,7 +433,7 @@ void main() {
       await tester.pump(kPressTimeout);
 
       // Drag enough to move down the first item
-      await drag.moveBy(const Offset(0, 150));
+      await drag.moveBy(const Offset(0, 50));
       await tester.pump();
       await drag.up();
       await tester.pumpAndSettle();