Fix ScrollbarPainter thumbExtent calculation and add padding

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

@@ -76,11 +76,11 @@ class _CupertinoScrollbarState extends State<CupertinoScrollbar> with TickerProv
   void didChangeDependencies() {
     super.didChangeDependencies();
     _textDirection = Directionality.of(context);
-    _painter = _buildCupertinoScrollbarPainter();
+    _painter = _buildCupertinoScrollbarPainter(MediaQuery.of(context).padding);
   }
 
   /// Returns a [ScrollbarPainter] visually styled like the iOS scrollbar.
-  ScrollbarPainter _buildCupertinoScrollbarPainter() {
+  ScrollbarPainter _buildCupertinoScrollbarPainter(EdgeInsets padding) {
     return ScrollbarPainter(
       color: _kScrollbarColor,
       textDirection: _textDirection,
@@ -89,6 +89,7 @@ class _CupertinoScrollbarState extends State<CupertinoScrollbar> with TickerProv
       mainAxisMargin: _kScrollbarMainAxisMargin,
       crossAxisMargin: _kScrollbarCrossAxisMargin,
       radius: _kScrollbarRadius,
+      padding: padding,
       minLength: _kScrollbarMinLength,
       minOverscrollLength: _kScrollbarMinOverscrollLength,
     );

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

@@ -93,17 +93,18 @@ class _ScrollbarState extends State<Scrollbar> with TickerProviderStateMixin {
       case TargetPlatform.fuchsia:
         _themeColor = theme.highlightColor.withOpacity(1.0);
         _textDirection = Directionality.of(context);
-        _materialPainter = _buildMaterialScrollbarPainter();
+        _materialPainter = _buildMaterialScrollbarPainter(MediaQuery.of(context).padding);
         break;
     }
   }
 
-  ScrollbarPainter _buildMaterialScrollbarPainter() {
+  ScrollbarPainter _buildMaterialScrollbarPainter(EdgeInsets padding) {
     return ScrollbarPainter(
         color: _themeColor,
         textDirection: _textDirection,
         thickness: _kScrollbarThickness,
         fadeoutOpacityAnimation: _fadeoutOpacityAnimation,
+        padding: 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 must not be null and must be 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 must not be null and must be 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].
@@ -98,12 +100,23 @@ abstract class ScrollMetrics {
   /// The quantity of visible content.
   ///
   /// 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 height of the viewport. The value will always be greater than or equal
+  /// to the size of the viewport.
+  ///
+  /// The implementation assumes [maxScrollExtent] is greater than or equal to
+  /// [minScrollExtent] and thus the value will always be greater than or equal
+  /// to [viewportDimension].
+  ///
+  /// See also:
+  ///
+  /// * [ViewportOffset.applyContentDimensions]
   double get extentInside {
+    assert(maxScrollExtent != null);
+    assert(minScrollExtent != null);
+    assert(minScrollExtent <= maxScrollExtent);
     return math.min(pixels, maxScrollExtent) -
            math.max(pixels, minScrollExtent) +
-           math.min(viewportDimension, maxScrollExtent - minScrollExtent);
+           viewportDimension;
   }
 
   /// The quantity of content conceptually "below" the currently visible content

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;