Fix "node._relayoutBoundary == _relayoutBoundary" crash (#44490)

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

@@ -1429,7 +1429,7 @@ abstract class RenderObject extends AbstractNode with DiagnosticableTreeMixin im
   static bool debugCheckingIntrinsics = false;
   bool _debugSubtreeRelayoutRootAlreadyMarkedNeedsLayout() {
     if (_relayoutBoundary == null)
-      return true; // we haven't yet done layout even once, so there's nothing for us to do
+      return true; // we don't know where our relayout boundary is yet
     RenderObject node = this;
     while (node != _relayoutBoundary) {
       assert(node._relayoutBoundary == _relayoutBoundary);
@@ -1677,6 +1677,14 @@ abstract class RenderObject extends AbstractNode with DiagnosticableTreeMixin im
       return;
     }
     _constraints = constraints;
+    if (_relayoutBoundary != null && relayoutBoundary != _relayoutBoundary) {
+      // The local relayout boundary has changed, must notify children in case
+      // they also need updating. Otherwise, they will be confused about what
+      // their actual relayout boundary is later.
+      visitChildren((RenderObject child) {
+        child._cleanRelayoutBoundary();
+      });
+    }
     _relayoutBoundary = relayoutBoundary;
     assert(!_debugMutationsLocked);
     assert(!_doingThisLayoutWithCallback);

packages/flutter/test/rendering/repaint_boundary_2_test.dart

@@ -0,0 +1,63 @@
+// Copyright 2018 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+import 'package:flutter_test/flutter_test.dart';
+import 'package:flutter/widgets.dart';
+
+void main() {
+  testWidgets('repaint boundary with constraint changes', (WidgetTester tester) async {
+    // Regression test for as https://github.com/flutter/flutter/issues/39151.
+    await tester.pumpWidget(const RelayoutBoundariesCrash());
+    tester.state<RelayoutBoundariesCrashState>(find.byType(RelayoutBoundariesCrash))._toggleMode();
+    await tester.pump();
+  });
+}
+
+class RelayoutBoundariesCrash extends StatefulWidget {
+  const RelayoutBoundariesCrash({Key key}) : super(key: key);
+
+  @override
+  RelayoutBoundariesCrashState createState() => RelayoutBoundariesCrashState();
+}
+
+class RelayoutBoundariesCrashState extends State<RelayoutBoundariesCrash> {
+  bool _mode = true;
+
+  void _toggleMode() {
+    setState(() {
+      _mode = !_mode;
+    });
+  }
+
+  @override
+  Widget build(BuildContext context) {
+    return Center(
+      child: SizedBox(
+        // when _mode is true, constraints are tight, otherwise constraints are loose
+        width: !_mode ? 100.0 : null,
+        height: !_mode ? 100.0 : null,
+        child: LayoutBuilder(
+          builder: (BuildContext context, BoxConstraints constraints) {
+            // Make the outer SizedBoxes relayout without making the Placeholders relayout.
+            final double dimension = !_mode ? 10.0 : 20.0;
+            return Column(
+              children: <Widget>[
+                SizedBox(
+                  width: dimension,
+                  height: dimension,
+                  child: const Placeholder(),
+                ),
+                SizedBox(
+                  width: dimension,
+                  height: dimension,
+                  child: const Placeholder(),
+                ),
+              ],
+            );
+          },
+        ),
+      ),
+    );
+  }
+}