Update scrollbar for hover events (#90636)

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

@@ -462,30 +462,38 @@ class ScrollbarPainter extends ChangeNotifier implements CustomPainter {
         break;
     }
 
+    // Whether we paint or not, calculating these rects allows us to hit test
+    // when the scrollbar is transparent.
     _trackRect = trackOffset & trackSize;
+    _thumbRect = Offset(x, y) & thumbSize;
+
+    // Paint if the opacity dictates visibility
+    if (fadeoutOpacityAnimation.value != 0.0) {
+      // Track
       canvas.drawRect(_trackRect!, _paintTrack());
+      // Track Border
       canvas.drawLine(
         trackOffset,
         Offset(trackOffset.dx, trackOffset.dy + _trackExtent),
         _paintTrack(isBorder: true),
       );
-
-    _thumbRect = Offset(x, y) & thumbSize;
-
       if (radius != null) {
-      canvas.drawRRect(RRect.fromRectAndRadius(_thumbRect!, radius!), _paintThumb);
+        // Rounded rect thumb
+        canvas.drawRRect(
+            RRect.fromRectAndRadius(_thumbRect!, radius!), _paintThumb);
         return;
       }
-
       if (shape == null) {
+        // Square thumb
         canvas.drawRect(_thumbRect!, _paintThumb);
         return;
       }
-
+      // Custom-shaped thumb
       final Path outerPath = shape!.getOuterPath(_thumbRect!);
       canvas.drawPath(outerPath, _paintThumb);
       shape!.paint(canvas, _thumbRect!);
     }
+  }
 
   double _thumbExtent() {
     // Thumb extent reflects fraction of content visible, as long as this
@@ -573,7 +581,6 @@ class ScrollbarPainter extends ChangeNotifier implements CustomPainter {
   void paint(Canvas canvas, Size size) {
     if (_lastAxisDirection == null
         || _lastMetrics == null
-        || fadeoutOpacityAnimation.value == 0.0
         || _lastMetrics!.maxScrollExtent <= _lastMetrics!.minScrollExtent)
       return;
 
@@ -606,7 +613,7 @@ class ScrollbarPainter extends ChangeNotifier implements CustomPainter {
   /// be used.
   bool hitTestInteractive(Offset position, PointerDeviceKind kind, { bool forHover = false }) {
     if (_thumbRect == null) {
-      // We have never painted the scrollbar, so we do not know where it will be.
+      // We have not computed the scrollbar position yet.
       return false;
     }
 

packages/flutter/test/widgets/scrollbar_test.dart

@@ -883,6 +883,41 @@ void main() {
     );
   });
 
+  testWidgets('Scrollbar will show on hover without needing to scroll first for metrics', (WidgetTester tester) async {
+    await tester.pumpWidget(
+      const Directionality(
+        textDirection: TextDirection.ltr,
+        child: MediaQuery(
+          data: MediaQueryData(),
+          child: RawScrollbar(
+            child: SingleChildScrollView(
+              child: SizedBox(width: 4000.0, height: 4000.0),
+            ),
+          ),
+        ),
+      ),
+    );
+    await tester.pump();
+
+    // Hover over scrollbar with mouse. Even though we have not scrolled, the
+    // ScrollMetricsNotification will have informed the Scrollbar's hit testing.
+    final TestGesture mouseGesture = await tester.createGesture(kind: ui.PointerDeviceKind.mouse);
+    await mouseGesture.addPointer();
+    addTearDown(mouseGesture.removePointer);
+    await mouseGesture.moveTo(const Offset(794.0, 5.0));
+    await tester.pumpAndSettle();
+    // Scrollbar should have appeared in response to hover event.
+    expect(
+      find.byType(RawScrollbar),
+      paints
+        ..rect(rect: const Rect.fromLTRB(794.0, 0.0, 800.0, 600.0))
+        ..rect(
+          rect: const Rect.fromLTRB(794.0, 0.0, 800.0, 90.0),
+          color: const Color(0x66BCBCBC),
+        ),
+    );
+  });
+
   testWidgets('Scrollbar thumb can be dragged', (WidgetTester tester) async {
     final ScrollController scrollController = ScrollController();
     await tester.pumpWidget(