Do not crash on LeaderLayer.applyTransform after retained rendering (#96144)

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

@@ -2234,7 +2234,7 @@ class LeaderLayer extends ContainerLayer {
   void attach(Object owner) {
     super.attach(owner);
     assert(link._leader == null);
-    _lastOffset = null;
+    assert(_debugSetLastOffset(null));
     link._leader = this;
   }
 
@@ -2242,7 +2242,7 @@ class LeaderLayer extends ContainerLayer {
   void detach() {
     assert(link._leader == this);
     link._leader = null;
-    _lastOffset = null;
+    assert(_debugSetLastOffset(null));
     super.detach();
   }
 
@@ -2251,7 +2251,17 @@ class LeaderLayer extends ContainerLayer {
   /// This is reset to null when the layer is attached or detached, to help
   /// catch cases where the follower layer ends up before the leader layer, but
   /// not every case can be detected.
-  Offset? _lastOffset;
+  Offset? _debugLastOffset;
+
+  bool _debugSetLastOffset(Offset? offset) {
+    bool result = false;
+    assert(() {
+      _debugLastOffset = offset;
+      result = true;
+      return true;
+    }());
+    return result;
+  }
 
   @override
   bool findAnnotations<S extends Object>(AnnotationResult<S> result, Offset localPosition, { required bool onlyFirst }) {
@@ -2261,14 +2271,14 @@ class LeaderLayer extends ContainerLayer {
   @override
   void addToScene(ui.SceneBuilder builder) {
     assert(offset != null);
-    _lastOffset = offset;
-    if (_lastOffset != Offset.zero)
+    assert(_debugSetLastOffset(offset));
+    if (offset != Offset.zero)
       engineLayer = builder.pushTransform(
-        Matrix4.translationValues(_lastOffset!.dx, _lastOffset!.dy, 0.0).storage,
+        Matrix4.translationValues(offset.dx, offset.dy, 0.0).storage,
         oldLayer: _engineLayer as ui.TransformEngineLayer?,
       );
     addChildrenToScene(builder);
-    if (_lastOffset != Offset.zero)
+    if (offset != Offset.zero)
       builder.pop();
   }
 
@@ -2281,9 +2291,8 @@ class LeaderLayer extends ContainerLayer {
   /// children.
   @override
   void applyTransform(Layer? child, Matrix4 transform) {
-    assert(_lastOffset != null);
-    if (_lastOffset != Offset.zero)
-      transform.translate(_lastOffset!.dx, _lastOffset!.dy);
+    if (offset != Offset.zero)
+      transform.translate(offset.dx, offset.dy);
   }
 
   @override
@@ -2499,7 +2508,7 @@ class FollowerLayer extends ContainerLayer {
       'Linked LeaderLayer anchor is not in the same layer tree as the FollowerLayer.',
     );
     assert(
-      leader._lastOffset != null,
+      leader._debugLastOffset != null,
       'LeaderLayer anchor must come before FollowerLayer in paint order, but the reverse was true.',
     );
 

packages/flutter/test/rendering/layers_test.dart

@@ -218,6 +218,36 @@ void main() {
     expect(leaderLayer.debugSubtreeNeedsAddToScene, false);
   });
 
+  test('LeaderLayer.applyTransform can be called after retained rendering', () {
+    void expectTransform(RenderObject leader) {
+      final LeaderLayer leaderLayer = leader.debugLayer! as LeaderLayer;
+      final Matrix4 expected = Matrix4.identity()
+        ..translate(leaderLayer.offset.dx, leaderLayer.offset.dy);
+      final Matrix4 transformed = Matrix4.identity();
+      leaderLayer.applyTransform(null, transformed);
+      expect(transformed, expected);
+    }
+
+    final LayerLink link = LayerLink();
+    late RenderLeaderLayer leader;
+    final RenderRepaintBoundary root = RenderRepaintBoundary(
+      child:RenderRepaintBoundary(
+        child: leader = RenderLeaderLayer(link: link),
+      ),
+    );
+    layout(root, phase: EnginePhase.composite);
+
+    expectTransform(leader);
+
+    // Causes a repaint, but the LeaderLayer of RenderLeaderLayer will be added
+    // as retained and LeaderLayer.addChildrenToScene will not be called.
+    root.markNeedsPaint();
+    pumpFrame(phase: EnginePhase.composite);
+
+    // The LeaderLayer.applyTransform call shouldn't crash.
+    expectTransform(leader);
+  });
+
   test('depthFirstIterateChildren', () {
     final ContainerLayer a = ContainerLayer();
     final ContainerLayer b = ContainerLayer();