fix a RenderBox.size access exception (#74402)

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

@@ -1942,7 +1942,8 @@ abstract class RenderBox extends RenderObject {
       final Size? _size = this._size;
       if (_size is _DebugSize) {
         assert(_size._owner == this);
-        if (RenderObject.debugActiveLayout != null) {
+        if (RenderObject.debugActiveLayout != null &&
+            !RenderObject.debugActiveLayout!.debugDoingThisLayoutWithCallback) {
           assert(
             debugDoingThisResize || debugDoingThisLayout || _computingThisDryLayout ||
               (RenderObject.debugActiveLayout == parent && _size._canBeUsedByParent),

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

@@ -1445,6 +1445,9 @@ abstract class RenderObject extends AbstractNode with DiagnosticableTreeMixin im
   bool _needsLayout = true;
 
   RenderObject? _relayoutBoundary;
+
+  /// Whether [invokeLayoutCallback] for this render object is currently running.
+  bool get debugDoingThisLayoutWithCallback => _doingThisLayoutWithCallback;
   bool _doingThisLayoutWithCallback = false;
 
   /// The layout constraints most recently supplied by the parent.

packages/flutter/test/widgets/layout_builder_test.dart

@@ -663,6 +663,38 @@ void main() {
     expect(spy.performLayoutCount, 3);
     expect(spy.performResizeCount, 2);
   });
+
+  testWidgets('LayoutBuilder descendant widget can access [RenderBox.size] when rebuilding during layout', (WidgetTester tester) async {
+    Size? childSize;
+    int buildCount = 0;
+
+    Future<void> pumpTestWidget(Size size) async {
+      await tester.pumpWidget(
+        // Center is used to give the SizedBox the power to determine constraints for LayoutBuilder
+        Center(
+          child: SizedBox.fromSize(
+            size: size,
+            child: LayoutBuilder(builder: (BuildContext context, BoxConstraints constraints) {
+              buildCount++;
+              if (buildCount > 1) {
+                final _RenderLayoutSpy spy = tester.renderObject(find.byType(_LayoutSpy));
+                childSize = spy.size;
+              }
+              return ColoredBox(
+                color: const Color(0xffffffff),
+                child: _LayoutSpy(),
+              );
+            }),
+          ),
+        ),
+      );
+    }
+
+    await pumpTestWidget(const Size(10.0, 10.0));
+    expect(childSize, isNull);
+    await pumpTestWidget(const Size(10.0, 10.0));
+    expect(childSize, const Size(10.0, 10.0));
+  });
 }
 
 class _LayoutSpy extends LeafRenderObjectWidget {