Don't show scrollbar if there isn't enough content (#34175)

packages/flutter/lib/src/cupertino/scrollbar.dart

@@ -103,6 +103,11 @@ class _CupertinoScrollbarState extends State<CupertinoScrollbar> with TickerProv
   }
 
   bool _handleScrollNotification(ScrollNotification notification) {
+    final ScrollMetrics metrics = notification.metrics;
+    if (metrics.maxScrollExtent <= metrics.minScrollExtent) {
+      return false;
+    }
+
     if (notification is ScrollUpdateNotification ||
         notification is OverscrollNotification) {
       // Any movements always makes the scrollbar start showing up.

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

@@ -109,6 +109,11 @@ class _ScrollbarState extends State<Scrollbar> with TickerProviderStateMixin {
   }
 
   bool _handleScrollNotification(ScrollNotification notification) {
+    final ScrollMetrics metrics = notification.metrics;
+    if (metrics.maxScrollExtent <= metrics.minScrollExtent) {
+      return false;
+    }
+
     // iOS sub-delegates to the CupertinoScrollbar instead and doesn't handle
     // scroll notifications here.
     if (_currentPlatform != TargetPlatform.iOS

packages/flutter/test/cupertino/scrollbar_paint_test.dart

@@ -77,7 +77,7 @@ void main() {
     final TestGesture gesture = await tester.startGesture(tester.getCenter(find.byType(ListView)));
     await gesture.moveBy(_kGestureOffset);
     // Move back to original position.
-    await gesture.moveBy(Offset.zero.translate(-_kGestureOffset.dx, -_kGestureOffset.dy));
+    await gesture.moveBy(Offset(-_kGestureOffset.dx, -_kGestureOffset.dy));
     await tester.pump();
     await tester.pump(const Duration(milliseconds: 500));
 
@@ -96,4 +96,44 @@ void main() {
       ),
     ));
   });
+
+  testWidgets("should not paint when there isn't enough space", (WidgetTester tester) async {
+    await tester.pumpWidget(
+      CupertinoApp(
+        home: MediaQuery(
+          data: const MediaQueryData(
+            padding: EdgeInsets.fromLTRB(0, 20, 0, 34),
+          ),
+          child: CupertinoPageScaffold(
+            navigationBar: const CupertinoNavigationBar(
+              middle: Text('Title'),
+              backgroundColor: Color(0x11111111),
+            ),
+            child: CupertinoScrollbar(
+              child: ListView(
+                physics: const AlwaysScrollableScrollPhysics(parent: BouncingScrollPhysics()),
+                children: const <Widget> [SizedBox(width: 10, height: 10)],
+              ),
+            ),
+          ),
+        ),
+      ),
+    );
+    final TestGesture gesture = await tester.startGesture(tester.getCenter(find.byType(ListView)));
+    await gesture.moveBy(_kGestureOffset);
+    // Move back to original position.
+    await gesture.moveBy(Offset(-_kGestureOffset.dx, -_kGestureOffset.dy));
+    await tester.pump();
+    await tester.pump(const Duration(milliseconds: 500));
+
+    expect(find.byType(CupertinoScrollbar), isNot(paints..rrect()));
+
+    // The scrollbar should not appear even when overscrolled.
+    final TestGesture overscrollGesture = await tester.startGesture(tester.getCenter(find.byType(ListView)));
+    await overscrollGesture.moveBy(_kGestureOffset);
+
+    await tester.pump();
+    await tester.pump(const Duration(milliseconds: 500));
+    expect(find.byType(CupertinoScrollbar), isNot(paints..rrect()));
+  });
 }

packages/flutter/test/material/scrollbar_paint_test.dart

@@ -77,4 +77,32 @@ void main() {
       ),
     ));
   });
+
+  testWidgets("should not paint when there isn't enough space", (WidgetTester tester) async {
+    await tester.pumpWidget(MaterialApp(
+      home: MediaQuery(
+        data: const MediaQueryData(
+          padding: EdgeInsets.fromLTRB(0, 20, 0, 34)
+        ),
+        child: Scaffold(
+          appBar: AppBar(title: const Text('Title')),
+          body: Scrollbar(
+            child: ListView(
+              children: const <Widget>[SizedBox(width: 40, height: 40)]
+            ),
+          ),
+        ),
+      ),
+    ));
+
+    final TestGesture gesture = await tester.startGesture(tester.getCenter(find.byType(ListView)));
+    // On Android it should not overscroll.
+    await gesture.moveBy(const Offset(0, 100));
+    // Trigger fade in animation.
+    await tester.pump();
+    await tester.pump(const Duration(milliseconds: 500));
+
+    expect(find.byType(Scrollbar), isNot(paints..rect()));
+  });
+
 }

packages/flutter/test/material/scrollbar_test.dart

@@ -115,8 +115,9 @@ void main() {
     final List<Invocation> invocations = <Invocation>[];
     final TestCanvas canvas = TestCanvas(invocations);
     scrollPainter.paint(canvas, const Size(10.0, 100.0));
-    final Rect thumbRect = invocations.single.positionalArguments[0];
-    expect(thumbRect.isFinite, isTrue);
+
+    // Scrollbar is not supposed to draw anything if there isn't enough content.
+    expect(invocations.isEmpty, isTrue);
   });
 
   testWidgets('Adaptive scrollbar', (WidgetTester tester) async {