Fix thumb size calculation (#36887)

Cupertino's thumb would jump up/down in size at the overscroll boundary, and this fixes it.

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

@@ -214,6 +214,7 @@ class ScrollbarPainter extends ChangeNotifier implements CustomPainter {
       trackExtent * fractionVisible
     );
 
+    final double fractionOverscrolled = 1.0 - extentInside / _lastMetrics.viewportDimension;
     final double safeMinLength = math.min(minLength, trackExtent);
     final double newMinLength = (beforeExtent > 0 && afterExtent > 0)
       // Thumb extent is no smaller than minLength if scrolling normally.
@@ -229,7 +230,7 @@ class ScrollbarPainter extends ChangeNotifier implements CustomPainter {
       // [0.8, 1.0] to [0.0, 1.0], so 0% to 20% of overscroll will produce
       // values for the thumb that range between minLength and the smallest
       // possible value, minOverscrollLength.
-      : safeMinLength * ((fractionVisible - 0.8).clamp(0.0, 0.2) / 0.2);
+      : safeMinLength * (1.0 - fractionOverscrolled.clamp(0.0, 0.2) / 0.2);
 
     // The `thumbExtent` should be no greater than `trackSize`, otherwise
     // the scrollbar may scroll towards the wrong direction.

packages/flutter/test/widgets/scrollbar_test.dart

@@ -371,6 +371,77 @@ void main() {
     });
   });
 
+  testWidgets('thumb resizes gradually on overscroll', (WidgetTester tester) async {
+    const EdgeInsets padding = EdgeInsets.fromLTRB(1, 2, 3, 4);
+    const Size size = Size(60, 300);
+    final double scrollExtent = size.height * 10;
+    final ScrollMetrics metrics = defaultMetrics.copyWith(
+      minScrollExtent: 0,
+      maxScrollExtent: scrollExtent,
+      axisDirection: AxisDirection.down,
+      viewportDimension: size.height,
+    );
+
+    const double minOverscrollLength = 8.0;
+    final ScrollbarPainter p = _buildPainter(
+      padding: padding,
+      scrollMetrics: metrics,
+      minLength: 36.0,
+      minOverscrollLength: 8.0,
+    );
+
+    // No overscroll gives a full sized thumb.
+    p.update(
+      metrics.copyWith(
+        pixels: 0.0,
+      ),
+      AxisDirection.down,
+    );
+    p.paint(testCanvas, size);
+    final double fullThumbExtent = captureRect().height;
+    expect(fullThumbExtent, greaterThan(_kMinThumbExtent));
+
+    // Scrolling to the middle also gives a full sized thumb.
+    p.update(
+      metrics.copyWith(
+        pixels: scrollExtent / 2,
+      ),
+      AxisDirection.down,
+    );
+    p.paint(testCanvas, size);
+    expect(captureRect().height, closeTo(fullThumbExtent, .000001));
+
+    // Scrolling just to the very end also gives a full sized thumb.
+    p.update(
+      metrics.copyWith(
+        pixels: scrollExtent,
+      ),
+      AxisDirection.down,
+    );
+    p.paint(testCanvas, size);
+    expect(captureRect().height, closeTo(fullThumbExtent, .000001));
+
+    // Scrolling just past the end shrinks the thumb slightly.
+    p.update(
+      metrics.copyWith(
+        pixels: scrollExtent * 1.001,
+      ),
+      AxisDirection.down,
+    );
+    p.paint(testCanvas, size);
+    expect(captureRect().height, closeTo(fullThumbExtent, 2.0));
+
+    // Scrolling way past the end shrinks the thumb to minimum.
+    p.update(
+      metrics.copyWith(
+        pixels: double.infinity,
+      ),
+      AxisDirection.down,
+    );
+    p.paint(testCanvas, size);
+    expect(captureRect().height, minOverscrollLength);
+  });
+
   test('should scroll towards the right direction',
     () {
       const Size size = Size(60, 80);