Allow current leader of layerlink to detach before the previous leade… (#96810)

packages/flutter/lib/src/rendering/layer.dart

@@ -2118,8 +2118,8 @@ class LayerLink {
     assert((){
       if (_leader != null) {
         _debugPreviousLeaders ??= <LeaderLayer>{};
-        _debugPreviousLeaders!.add(_leader!);
         _debugScheduleLeadersCleanUpCheck();
+        return _debugPreviousLeaders!.add(_leader!);
       }
       return true;
     }());
@@ -2127,14 +2127,10 @@ class LayerLink {
   }
 
   void _unregisterLeader(LeaderLayer leader) {
-    assert(_leader != null);
     if (_leader == leader) {
       _leader = null;
     } else {
-      assert((){
-        _debugPreviousLeaders!.remove(leader);
-        return true;
-      }());
+      assert(_debugPreviousLeaders!.remove(leader));
     }
   }
 

packages/flutter/test/rendering/layers_test.dart

@@ -176,6 +176,19 @@ void main() {
     expect(leaderLayer.link, link2);
   });
 
+  test('layer link attach/detach order should not crash app.', () {
+    renderer;
+    final LayerLink link = LayerLink();
+    final LeaderLayer leaderLayer1 = LeaderLayer(link: link);
+    final LeaderLayer leaderLayer2 = LeaderLayer(link: link);
+    final RenderView view = RenderView(configuration: const ViewConfiguration(), window: window);
+    leaderLayer1.attach(view);
+    leaderLayer2.attach(view);
+    leaderLayer2.detach();
+    leaderLayer1.detach();
+    expect(link.leader, isNull);
+  });
+
   test('leader layers not dirty when connected to follower layer', () {
     final ContainerLayer root = ContainerLayer()..attach(Object());
 

packages/flutter/test/widgets/framework_test.dart

@@ -5,6 +5,7 @@
 import 'package:flutter/foundation.dart';
 import 'package:flutter/gestures.dart';
 import 'package:flutter/material.dart';
+import 'package:flutter/rendering.dart';
 import 'package:flutter/services.dart';
 import 'package:flutter_test/flutter_test.dart';
 
@@ -1587,6 +1588,28 @@ void main() {
     expect(() => element.widget, throwsA(isA<TypeError>()));
   });
 
+  testWidgets('LayerLink can be swapped between parent and child container layers', (WidgetTester tester) async {
+    // Regression test for https://github.com/flutter/flutter/issues/96959.
+    final LayerLink link = LayerLink();
+    await tester.pumpWidget(_TestLeaderLayerWidget(
+        link: link,
+        child: const _TestLeaderLayerWidget(
+          child: Placeholder(),
+        )
+    ));
+    expect(tester.takeException(), isNull);
+
+    // Swaps the layer link.
+    await tester.pumpWidget(_TestLeaderLayerWidget(
+        child: _TestLeaderLayerWidget(
+          link: link,
+          child: const Placeholder(),
+        ),
+    ));
+    expect(tester.takeException(), isNull);
+
+  });
+
   testWidgets('Deactivate and activate are called correctly', (WidgetTester tester) async {
     final List<String> states = <String>[];
     Widget build([Key? key]) {
@@ -2031,3 +2054,53 @@ class _EmptyElement extends Element {
   @override
   void performRebuild() {}
 }
+
+class _TestLeaderLayerWidget extends SingleChildRenderObjectWidget {
+  const _TestLeaderLayerWidget({
+    Key? key,
+    this.link,
+    Widget? child,
+  }) : super(key: key, child: child);
+  final LayerLink? link;
+
+  @override
+  _RenderTestLeaderLayerWidget createRenderObject(BuildContext context) {
+    return _RenderTestLeaderLayerWidget(
+      link: link,
+    );
+  }
+
+  @override
+  void updateRenderObject(BuildContext context, _RenderTestLeaderLayerWidget renderObject) {
+    renderObject.link = link;
+  }
+}
+
+class _RenderTestLeaderLayerWidget extends RenderProxyBox {
+  _RenderTestLeaderLayerWidget({
+    LayerLink? link,
+    RenderBox? child,
+  }) : _link = link,
+        super(child);
+
+  LayerLink? get link => _link;
+  LayerLink? _link;
+  set link(LayerLink? value) {
+    if (_link == value) {
+      return;
+    }
+    _link = value;
+    markNeedsPaint();
+  }
+
+  @override
+  bool get isRepaintBoundary => true;
+
+  @override
+  void paint(PaintingContext context, Offset offset) {
+    super.paint(context, offset);
+    if (_link != null) {
+      context.pushLayer(LeaderLayer(link: _link!, offset: offset),(_, __){}, Offset.zero);
+    }
+  }
+}