Fixes to CupertinoScrollbar minLength (#20101)

Add minOverscrollLength to make CupertinoScrollbar thumb correct size in long scroll views.

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

@@ -11,7 +11,8 @@ const Color _kScrollbarColor = Color(0x99777777);
 const double _kScrollbarThickness = 2.5;
 const double _kScrollbarMainAxisMargin = 4.0;
 const double _kScrollbarCrossAxisMargin = 2.5;
-const double _kScrollbarMinLength = 4.0;
+const double _kScrollbarMinLength = 36.0;
+const double _kScrollbarMinOverscrollLength = 8.0;
 const Radius _kScrollbarRadius = Radius.circular(1.25);
 const Duration _kScrollbarTimeToFade = Duration(milliseconds: 50);
 const Duration _kScrollbarFadeDuration = Duration(milliseconds: 250);
@@ -89,6 +90,7 @@ class _CupertinoScrollbarState extends State<CupertinoScrollbar> with TickerProv
       crossAxisMargin: _kScrollbarCrossAxisMargin,
       radius: _kScrollbarRadius,
       minLength: _kScrollbarMinLength,
+      minOverscrollLength: _kScrollbarMinOverscrollLength,
     );
   }
 

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

@@ -47,6 +47,7 @@ class ScrollbarPainter extends ChangeNotifier implements CustomPainter {
     this.crossAxisMargin = 0.0,
     this.radius,
     this.minLength = _kMinThumbExtent,
+    this.minOverscrollLength = _kMinThumbExtent,
   }) : assert(color != null),
        assert(textDirection != null),
        assert(thickness != null),
@@ -86,10 +87,14 @@ class ScrollbarPainter extends ChangeNotifier implements CustomPainter {
   final Radius radius;
 
   /// The smallest size the scrollbar can shrink to when the total scrollable
-  /// extent is large and the current visible viewport is small. Mustn't be
-  /// null.
+  /// extent is large and the current visible viewport is small, and the
+  /// viewport is not overscrolled. Mustn't be null.
   final double minLength;
 
+  /// The smallest size the scrollbar can shrink to when viewport is
+  /// overscrolled. Mustn't be null.
+  final double minOverscrollLength;
+
   ScrollMetrics _lastMetrics;
   AxisDirection _lastAxisDirection;
 
@@ -152,13 +157,40 @@ class ScrollbarPainter extends ChangeNotifier implements CustomPainter {
     void painter(Canvas canvas, Size size, double thumbOffset, double thumbExtent),
   ) {
     // Establish the minimum size possible.
-    double thumbExtent = math.min(viewport, minLength);
+    double thumbExtent = math.min(viewport, minOverscrollLength);
+
     if (before + inside + after > 0.0) {
+      // Thumb extent reflects fraction of content visible, as long as this
+      // isn't less than the absolute minimum size.
       final double fractionVisible = inside / (before + inside + after);
       thumbExtent = math.max(
         thumbExtent,
         viewport * fractionVisible - 2 * mainAxisMargin,
       );
+      // Thumb extent is no smaller than minLength if scrolling normally.
+      if (before != 0.0 && after != 0.0) {
+        thumbExtent = math.max(
+          minLength,
+          thumbExtent,
+        );
+      }
+      // User is overscrolling. Thumb extent can be less than minLength
+      // but no smaller than minOverscrollLength. We can't use the
+      // fractionVisible to produce intermediate values between minLength and
+      // minOverscrollLength when the user is transitioning from regular
+      // scrolling to overscrolling, so we instead use the percentage of the
+      // content that is still in the viewport to determine the size of the
+      // thumb. iOS behavior appears to have the thumb reach its minimum size
+      // with ~20% of overscroll. We map the percentage of minLength from
+      // [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.
+      else {
+        thumbExtent = math.max(
+          thumbExtent,
+          minLength * (((inside / viewport) - 0.8) / 0.2),
+        );
+      }
     }
 
     final double fractionPast = before / (before + after);

packages/flutter/test/cupertino/scrollbar_test.dart

@@ -42,4 +42,26 @@ void main() {
       color: const Color(0x15777777),
     ));
   });
+
+  testWidgets('Scrollbar is not smaller than minLength with large scroll views',
+          (WidgetTester tester) async {
+    await tester.pumpWidget(new Directionality(
+      textDirection: TextDirection.ltr,
+      child: new CupertinoScrollbar(
+        child: new SingleChildScrollView(
+          child: const SizedBox(width: 800.0, height: 20000.0),
+        ),
+      ),
+    ));
+    final TestGesture gesture = await tester.startGesture(tester.getCenter(find.byType(SingleChildScrollView)));
+    await gesture.moveBy(const Offset(0.0, -10.0));
+    await tester.pump();
+    await tester.pump(const Duration(milliseconds: 200));
+
+    // Height is 36.0.
+    final Rect scrollbarRect = Rect.fromLTWH(795.0, 4.28659793814433, 2.5, 36.0);
+    expect(find.byType(CupertinoScrollbar), paints..rrect(
+      rrect: RRect.fromRectAndRadius(scrollbarRect, const Radius.circular(1.25)),
+    ));
+  });
 }