Always push layer for RenderAnimatedOpacityMixin (#83145)

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

@@ -993,7 +993,7 @@ mixin RenderAnimatedOpacityMixin<T extends RenderObject> on RenderObjectWithChil
     _alpha = ui.Color.getAlphaFromOpacity(opacity.value);
     if (oldAlpha != _alpha) {
       final bool? didNeedCompositing = _currentlyNeedsCompositing;
-      _currentlyNeedsCompositing = _alpha! > 0 && _alpha! < 255;
+      _currentlyNeedsCompositing = _alpha! > 0;
       if (child != null && didNeedCompositing != _currentlyNeedsCompositing)
         markNeedsCompositingBitsUpdate();
       markNeedsPaint();
@@ -1010,12 +1010,6 @@ mixin RenderAnimatedOpacityMixin<T extends RenderObject> on RenderObjectWithChil
         layer = null;
         return;
       }
-      if (_alpha == 255) {
-        // No need to keep the layer. We'll create a new one if necessary.
-        layer = null;
-        context.paintChild(child!, offset);
-        return;
-      }
       assert(needsCompositing);
       layer = context.pushOpacity(offset, _alpha!, super.paint, oldLayer: layer as OpacityLayer?);
     }

packages/flutter/test/rendering/proxy_box_test.dart

@@ -306,7 +306,7 @@ void main() {
     expect(renderAnimatedOpacity.needsCompositing, false);
   });
 
-  test('RenderAnimatedOpacity does not composite if it is opaque', () {
+  test('RenderAnimatedOpacity does composite if it is opaque', () {
     final Animation<double> opacityAnimation = AnimationController(
       vsync: FakeTickerProvider(),
     )..value = 1.0;
@@ -318,7 +318,7 @@ void main() {
     );
 
     layout(renderAnimatedOpacity, phase: EnginePhase.composite);
-    expect(renderAnimatedOpacity.needsCompositing, false);
+    expect(renderAnimatedOpacity.needsCompositing, true);
   });
 
   test('RenderAnimatedOpacity reuses its layer', () {

packages/flutter/test/rendering/proxy_sliver_test.dart

@@ -102,7 +102,7 @@ void main() {
     expect(renderSliverAnimatedOpacity.needsCompositing, false);
   });
 
-  test('RenderSliverAnimatedOpacity does not composite if it is opaque', () {
+  test('RenderSliverAnimatedOpacity does composite if it is opaque', () {
     final Animation<double> opacityAnimation = AnimationController(
       vsync: FakeTickerProvider(),
     )..value = 1.0;
@@ -124,7 +124,7 @@ void main() {
     );
 
     layout(root, phase: EnginePhase.composite);
-    expect(renderSliverAnimatedOpacity.needsCompositing, false);
+    expect(renderSliverAnimatedOpacity.needsCompositing, true);
   });
 
   test('RenderSliverAnimatedOpacity reuses its layer', () {