[framework] ensure flexible space bar fades when scrolling. (#129527)

This was missing the actual opacity rendering, oops

packages/flutter/lib/src/material/flexible_space_bar.dart

@@ -451,4 +451,21 @@ class _RenderFlexibleSpaceHeaderOpacity extends RenderOpacity {
 
   @override
   bool get isRepaintBoundary => false;
+
+  @override
+  void paint(PaintingContext context, Offset offset) {
+    if (child == null) {
+      return;
+    }
+    if (opacity == 0) {
+      layer = null;
+      return;
+    }
+    assert(needsCompositing);
+    layer = context.pushOpacity(offset, (opacity * 255).round(), super.paint, oldLayer: layer as OpacityLayer?);
+    assert(() {
+      layer!.debugCreator = debugCreator;
+      return true;
+    }());
+  }
 }

packages/flutter/test/material/flexible_space_bar_test.dart

@@ -801,14 +801,29 @@ void main() {
     ));
 
     expect(RenderRebuildTracker.count, 1);
+    expect(
+      tester.layers.lastWhere((Layer element) => element is OpacityLayer),
+      isA<OpacityLayer>().having((OpacityLayer p0) => p0.alpha, 'alpha', 255),
+    );
 
     // We drag up to fully collapse the space bar.
-    for (int i = 0; i < 20; i++) {
+    for (int i = 0; i < 9; i++) {
+      await tester.drag(find.byKey(SubCategoryScreenView.scrollKey), const Offset(0, -50.0));
+      await tester.pumpAndSettle();
+    }
+
+    expect(
+      tester.layers.lastWhere((Layer element) => element is OpacityLayer),
+      isA<OpacityLayer>().having((OpacityLayer p0) => p0.alpha, 'alpha', lessThan(255)),
+    );
+
+    for (int i = 0; i < 11; i++) {
       await tester.drag(find.byKey(SubCategoryScreenView.scrollKey), const Offset(0, -50.0));
       await tester.pumpAndSettle();
     }
 
     expect(RenderRebuildTracker.count, greaterThan(1));
+    expect(tester.layers.whereType<OpacityLayer>(), isEmpty);
   });
 }