Fix ScrollbarPainter thumbExtent calculation and add padding (#31763)

- Fixed extentInside calculation in ScrollMetrics
- Added asserts to extentInside getter, as well as ScrollPosition.applyContentDimensions to enforce minScrollExtent <= maxScrollExtent
- Added padding to ScrollbarPainter, updated implementation. Took care of some edge cases.
- Changed some scroll bar constants on Cupertino side.

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

@@ -8,15 +8,22 @@ import 'package:flutter/widgets.dart';
 
 // All values eyeballed.
 const Color _kScrollbarColor = Color(0x99777777);
-const double _kScrollbarThickness = 2.5;
-const double _kScrollbarMainAxisMargin = 4.0;
-const double _kScrollbarCrossAxisMargin = 2.5;
 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);
 
+// These values are measured using screenshots from an iPhone XR 12.1 simulator.
+const double _kScrollbarThickness = 2.5;
+// This is the amount of space from the top of a vertical scrollbar to the
+// top edge of the scrollable, measured when the vertical scrollbar overscrolls
+// to the top.
+// TODO(LongCatIsLooong): fix https://github.com/flutter/flutter/issues/32175
+const double _kScrollbarMainAxisMargin = 3.0;
+const double _kScrollbarCrossAxisMargin = 3.0;
+
+
 /// An iOS style scrollbar.
 ///
 /// A scrollbar indicates which portion of a [Scrollable] widget is actually
@@ -89,6 +96,7 @@ class _CupertinoScrollbarState extends State<CupertinoScrollbar> with TickerProv
       mainAxisMargin: _kScrollbarMainAxisMargin,
       crossAxisMargin: _kScrollbarCrossAxisMargin,
       radius: _kScrollbarRadius,
+      padding: MediaQuery.of(context).padding,
       minLength: _kScrollbarMinLength,
       minOverscrollLength: _kScrollbarMinOverscrollLength,
     );

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

@@ -104,6 +104,7 @@ class _ScrollbarState extends State<Scrollbar> with TickerProviderStateMixin {
         textDirection: _textDirection,
         thickness: _kScrollbarThickness,
         fadeoutOpacityAnimation: _fadeoutOpacityAnimation,
+        padding: MediaQuery.of(context).padding,
       );
   }
 

packages/flutter/lib/src/rendering/viewport_offset.dart

@@ -123,8 +123,9 @@ abstract class ViewportOffset extends ChangeNotifier {
   /// Called when the viewport's content extents are established.
   ///
   /// The arguments are the minimum and maximum scroll extents respectively. The
-  /// minimum will be equal to or less than zero, the maximum will be equal to
-  /// or greater than zero.
+  /// minimum will be equal to or less than the maximum. In the case of slivers,
+  /// the minimum will be equal to or less than zero, the maximum will be equal
+  /// to or greater than zero.
   ///
   /// The maximum scroll extent has the viewport dimension subtracted from it.
   /// For instance, if there is 100.0 pixels of scrollable content, and the

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

@@ -60,14 +60,16 @@ abstract class ScrollMetrics {
   ///
   /// The actual [pixels] value might be [outOfRange].
   ///
-  /// This value can be negative infinity, if the scroll is unbounded.
+  /// This value should typically be non-null and less than or equal to
+  /// [maxScrollExtent]. It can be negative infinity, if the scroll is unbounded.
   double get minScrollExtent;
 
   /// The maximum in-range value for [pixels].
   ///
   /// The actual [pixels] value might be [outOfRange].
   ///
-  /// This value can be infinity, if the scroll is unbounded.
+  /// This value should typically be non-null and greater than or equal to
+  /// [minScrollExtent]. It can be infinity, if the scroll is unbounded.
   double get maxScrollExtent;
 
   /// The current scroll position, in logical pixels along the [axisDirection].
@@ -90,25 +92,27 @@ abstract class ScrollMetrics {
   /// [maxScrollExtent].
   bool get atEdge => pixels == minScrollExtent || pixels == maxScrollExtent;
 
-  /// The quantity of content conceptually "above" the currently visible content
-  /// of the viewport in the scrollable. This is the content above the content
-  /// described by [extentInside].
+  /// The quantity of content conceptually "above" the viewport in the scrollable.
+  /// This is the content above the content described by [extentInside].
   double get extentBefore => math.max(pixels - minScrollExtent, 0.0);
 
-  /// The quantity of visible content.
+  /// The quantity of content conceptually "inside" the viewport in the scrollable.
   ///
-  /// If [extentBefore] and [extentAfter] are non-zero, then this is typically
-  /// the height of the viewport. It could be less if there is less content
-  /// visible than the size of the viewport.
+  /// The value is typically the height of the viewport when [outOfRange] is false.
+  /// It could be less if there is less content visible than the size of the
+  /// viewport, such as when overscrolling.
+  ///
+  /// The value is always non-negative, and less than or equal to [viewportDimension].
   double get extentInside {
-    return math.min(pixels, maxScrollExtent) -
-           math.max(pixels, minScrollExtent) +
-           math.min(viewportDimension, maxScrollExtent - minScrollExtent);
+    return viewportDimension
+      // "above" overscroll value
+      - (minScrollExtent - pixels).clamp(0, viewportDimension)
+      // "below" overscroll value
+      - (pixels - maxScrollExtent).clamp(0, viewportDimension);
   }
 
-  /// The quantity of content conceptually "below" the currently visible content
-  /// of the viewport in the scrollable. This is the content below the content
-  /// described by [extentInside].
+  /// The quantity of content conceptually "below" the viewport in the scrollable.
+  /// This is the content below the content described by [extentInside].
   double get extentAfter => math.max(maxScrollExtent - pixels, 0.0);
 }
 

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

@@ -450,6 +450,9 @@ abstract class ScrollPosition extends ViewportOffset with ScrollMetrics {
     if (!nearEqual(_minScrollExtent, minScrollExtent, Tolerance.defaultTolerance.distance) ||
         !nearEqual(_maxScrollExtent, maxScrollExtent, Tolerance.defaultTolerance.distance) ||
         _didChangeViewportDimensionOrReceiveCorrection) {
+      assert(minScrollExtent != null);
+      assert(maxScrollExtent != null);
+      assert(minScrollExtent <= maxScrollExtent);
       _minScrollExtent = minScrollExtent;
       _maxScrollExtent = maxScrollExtent;
       _haveDimensions = true;