diff --git a/packages/flutter/lib/src/rendering/proxy_box.dart b/packages/flutter/lib/src/rendering/proxy_box.dart
index 3f9b68dd7a..93dbcb86a0 100644
--- a/packages/flutter/lib/src/rendering/proxy_box.dart
+++ b/packages/flutter/lib/src/rendering/proxy_box.dart
@@ -53,8 +53,10 @@ class RenderProxyBox extends RenderBox with RenderObjectWithChildMixin<RenderBox
 ///
 /// Use this mixin in situations where the proxying behavior
 /// of [RenderProxyBox] is desired but inheriting from [RenderProxyBox] is
-/// impractical (e.g. because you want to mix in other classes as well).
-// TODO(ianh): Remove this class once https://github.com/dart-lang/sdk/issues/31543 is fixed
+/// impractical (e.g. because you want to inherit from a different class).
+///
+/// If a class already inherits from [RenderProxyBox] and also uses this mixin,
+/// you can safely delete the use of the mixin.
 @optionalTypeArgs
 mixin RenderProxyBoxMixin<T extends RenderBox> on RenderBox, RenderObjectWithChildMixin<T> {
   @override
@@ -1095,7 +1097,7 @@ mixin RenderAnimatedOpacityMixin<T extends RenderObject> on RenderObjectWithChil
 ///
 /// This is a variant of [RenderOpacity] that uses an [Animation<double>] rather
 /// than a [double] to control the opacity.
-class RenderAnimatedOpacity extends RenderProxyBox with RenderProxyBoxMixin, RenderAnimatedOpacityMixin<RenderBox> {
+class RenderAnimatedOpacity extends RenderProxyBox with RenderAnimatedOpacityMixin<RenderBox> {
   /// Creates a partially transparent render object.
   ///
   /// The [opacity] argument must not be null.
diff --git a/packages/flutter/test/rendering/proxy_box_test.dart b/packages/flutter/test/rendering/proxy_box_test.dart
index b397a7eeb6..2a2ec4e3d0 100644
--- a/packages/flutter/test/rendering/proxy_box_test.dart
+++ b/packages/flutter/test/rendering/proxy_box_test.dart
@@ -963,6 +963,17 @@ void main() {
     expect(debugPaintClipOval(Clip.none), paintsExactlyCountTimes(#drawPath, 0));
     expect(debugPaintClipOval(Clip.none), paintsExactlyCountTimes(#drawParagraph, 0));
   });
+
+  test('RenderProxyBox behavior can be mixed in along with another base class', () {
+    final RenderFancyProxyBox fancyProxyBox = RenderFancyProxyBox(fancy: 6);
+    // Box has behavior from its base class:
+    expect(fancyProxyBox.fancyMethod(), 36);
+    // Box has behavior from RenderProxyBox:
+    expect(
+      fancyProxyBox.computeDryLayout(const BoxConstraints(minHeight: 8)),
+      const Size(0, 8),
+    );
+  });
 }
 
 class _TestRectClipper extends CustomClipper<Rect> {
@@ -1061,6 +1072,21 @@ class ConditionalRepaintBoundary extends RenderProxyBox {
 
 class TestOffsetLayerA extends OffsetLayer {}
 
+class RenderFancyBox extends RenderBox {
+  RenderFancyBox({required this.fancy}) : super();
+
+  late int fancy;
+
+  int fancyMethod() {
+    return fancy * fancy;
+  }
+}
+
+class RenderFancyProxyBox extends RenderFancyBox
+    with RenderObjectWithChildMixin<RenderBox>, RenderProxyBoxMixin<RenderBox> {
+  RenderFancyProxyBox({required super.fancy});
+}
+
 void expectAssertionError() {
   final FlutterErrorDetails errorDetails = TestRenderingFlutterBinding.instance.takeFlutterErrorDetails()!;
   final bool asserted = errorDetails.toString().contains('Failed assertion');