Fix scrollbar margins (#107172)

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

@@ -211,8 +211,10 @@ class ScrollbarPainter extends ChangeNotifier implements CustomPainter {
   /// Mustn't be null.
   final Animation<double> fadeoutOpacityAnimation;
 
-  /// Distance from the scrollbar's start and end to the edge of the viewport
-  /// in logical pixels. It affects the amount of available paint area.
+  /// Distance from the scrollbar thumb's start and end to the edge of the
+  /// viewport in logical pixels. It affects the amount of available paint area.
+  ///
+  /// The scrollbar track consumes this space.
   ///
   /// Mustn't be null and defaults to 0.
   double get mainAxisMargin => _mainAxisMargin;
@@ -230,6 +232,8 @@ class ScrollbarPainter extends ChangeNotifier implements CustomPainter {
   /// Distance from the scrollbar thumb to the nearest cross axis edge
   /// in logical pixels.
   ///
+  /// The scrollbar track consumes this space.
+  ///
   /// Must not be null and defaults to 0.
   double get crossAxisMargin => _crossAxisMargin;
   double _crossAxisMargin;
@@ -488,7 +492,7 @@ class ScrollbarPainter extends ChangeNotifier implements CustomPainter {
         trackSize = Size(thickness + 2 * crossAxisMargin, _trackExtent);
         x = crossAxisMargin + padding.left;
         y = _thumbOffset;
-        trackOffset = Offset(x - crossAxisMargin, mainAxisMargin + padding.top);
+        trackOffset = Offset(x - crossAxisMargin, padding.top);
         borderStart = trackOffset + Offset(trackSize.width, 0.0);
         borderEnd = Offset(trackOffset.dx + trackSize.width, trackOffset.dy + _trackExtent);
         break;
@@ -497,7 +501,7 @@ class ScrollbarPainter extends ChangeNotifier implements CustomPainter {
         trackSize = Size(thickness + 2 * crossAxisMargin, _trackExtent);
         x = size.width - thickness - crossAxisMargin - padding.right;
         y = _thumbOffset;
-        trackOffset = Offset(x - crossAxisMargin, mainAxisMargin + padding.top);
+        trackOffset = Offset(x - crossAxisMargin, padding.top);
         borderStart = trackOffset;
         borderEnd = Offset(trackOffset.dx, trackOffset.dy + _trackExtent);
         break;
@@ -506,7 +510,7 @@ class ScrollbarPainter extends ChangeNotifier implements CustomPainter {
         trackSize = Size(_trackExtent, thickness + 2 * crossAxisMargin);
         x = _thumbOffset;
         y = crossAxisMargin + padding.top;
-        trackOffset = Offset(mainAxisMargin + padding.left, y - crossAxisMargin);
+        trackOffset = Offset(padding.left, y - crossAxisMargin);
         borderStart = trackOffset + Offset(0.0, trackSize.height);
         borderEnd = Offset(trackOffset.dx + _trackExtent, trackOffset.dy + trackSize.height);
         break;
@@ -515,7 +519,7 @@ class ScrollbarPainter extends ChangeNotifier implements CustomPainter {
         trackSize = Size(_trackExtent, thickness + 2 * crossAxisMargin);
         x = _thumbOffset;
         y = size.height - thickness - crossAxisMargin - padding.bottom;
-        trackOffset = Offset(mainAxisMargin + padding.left, y - crossAxisMargin);
+        trackOffset = Offset(padding.left, y - crossAxisMargin);
         borderStart = trackOffset;
         borderEnd = Offset(trackOffset.dx + _trackExtent, trackOffset.dy);
         break;
@@ -564,12 +568,12 @@ class ScrollbarPainter extends ChangeNotifier implements CustomPainter {
         1.0);
 
     final double thumbExtent = math.max(
-      math.min(_trackExtent, minOverscrollLength),
-      _trackExtent * fractionVisible,
+      math.min(_traversableTrackExtent, minOverscrollLength),
+      _traversableTrackExtent * fractionVisible,
     );
 
     final double fractionOverscrolled = 1.0 - _lastMetrics!.extentInside / _lastMetrics!.viewportDimension;
-    final double safeMinLength = math.min(minLength, _trackExtent);
+    final double safeMinLength = math.min(minLength, _traversableTrackExtent);
     final double newMinLength = (_beforeExtent > 0 && _afterExtent > 0)
       // Thumb extent is no smaller than minLength if scrolling normally.
       ? safeMinLength
@@ -588,7 +592,8 @@ class ScrollbarPainter extends ChangeNotifier implements CustomPainter {
 
     // The `thumbExtent` should be no greater than `trackSize`, otherwise
     // the scrollbar may scroll towards the wrong direction.
-    return clampDouble(thumbExtent, newMinLength, _trackExtent);
+    final double extent = clampDouble(thumbExtent, newMinLength, _traversableTrackExtent);
+    return extent;
   }
 
   @override
@@ -604,8 +609,10 @@ class ScrollbarPainter extends ChangeNotifier implements CustomPainter {
   double get _afterExtent => _isReversed ? _lastMetrics!.extentBefore : _lastMetrics!.extentAfter;
   // Padding of the thumb track.
   double get _mainAxisPadding => _isVertical ? padding.vertical : padding.horizontal;
-  // The size of the thumb track.
-  double get _trackExtent => _lastMetrics!.viewportDimension - 2 * mainAxisMargin - _mainAxisPadding;
+  // The length of the painted track.
+  double get _trackExtent => _lastMetrics!.viewportDimension - _mainAxisPadding;
+  // The length of the track that is traversable by the thumb.
+  double get _traversableTrackExtent => _trackExtent - (2 * mainAxisMargin);
 
   // The total size of the scrollable content.
   double get _totalContentExtent {
@@ -621,7 +628,7 @@ class ScrollbarPainter extends ChangeNotifier implements CustomPainter {
   double getTrackToScroll(double thumbOffsetLocal) {
     assert(thumbOffsetLocal != null);
     final double scrollableExtent = _lastMetrics!.maxScrollExtent - _lastMetrics!.minScrollExtent;
-    final double thumbMovableExtent = _trackExtent - _thumbExtent();
+    final double thumbMovableExtent = _traversableTrackExtent - _thumbExtent();
 
     return scrollableExtent * thumbOffsetLocal / thumbMovableExtent;
   }
@@ -635,7 +642,7 @@ class ScrollbarPainter extends ChangeNotifier implements CustomPainter {
       ? clampDouble((metrics.pixels - metrics.minScrollExtent) / scrollableExtent, 0.0, 1.0)
       : 0;
 
-    return (_isReversed ? 1 - fractionPast : fractionPast) * (_trackExtent - thumbExtent);
+    return (_isReversed ? 1 - fractionPast : fractionPast) * (_traversableTrackExtent - thumbExtent);
   }
 
   @override
@@ -647,7 +654,7 @@ class ScrollbarPainter extends ChangeNotifier implements CustomPainter {
     }
 
     // Skip painting if there's not enough space.
-    if (_lastMetrics!.viewportDimension <= _mainAxisPadding || _trackExtent <= 0) {
+    if (_lastMetrics!.viewportDimension <= _mainAxisPadding || _traversableTrackExtent <= 0) {
       return;
     }
 
@@ -1342,15 +1349,20 @@ class RawScrollbar extends StatefulWidget {
   /// {@macro flutter.widgets.Scrollbar.scrollbarOrientation}
   final ScrollbarOrientation? scrollbarOrientation;
 
-  /// Distance from the scrollbar's start and end to the edge of the viewport
-  /// in logical pixels. It affects the amount of available paint area.
+  /// Distance from the scrollbar thumb's start or end to the nearest edge of
+  /// the viewport in logical pixels. It affects the amount of available
+  /// paint area.
+  ///
+  /// The scrollbar track consumes this space.
   ///
   /// Mustn't be null and defaults to 0.
   final double mainAxisMargin;
 
-  /// Distance from the scrollbar thumb side to the nearest cross axis edge
+  /// Distance from the scrollbar thumb's side to the nearest cross axis edge
   /// in logical pixels.
   ///
+  /// The scrollbar track consumes this space.
+  ///
   /// Must not be null and defaults to 0.
   final double crossAxisMargin;
 

packages/flutter/test/cupertino/scrollbar_test.dart

@@ -1230,11 +1230,11 @@ void main() {
       find.byType(CupertinoScrollbar),
       paints
         ..rect(
-          rect: const Rect.fromLTRB(0.0, 3.0, 9.0, 597.0),
+          rect: const Rect.fromLTRB(0.0, 0.0, 9.0, 600.0),
         )
         ..line(
-          p1: const Offset(9.0, 3.0),
-          p2: const Offset(9.0, 597.0),
+          p1: const Offset(9.0, 0.0),
+          p2: const Offset(9.0, 600.0),
           strokeWidth: 1.0,
         )
         ..rrect(

packages/flutter/test/material/scrollbar_theme_test.dart

@@ -179,12 +179,12 @@ void main() {
       find.byType(Scrollbar),
       paints
         ..rect(
-          rect: const Rect.fromLTRB(770.0, 10.0, 800.0, 590.0),
+          rect: const Rect.fromLTRB(770.0, 0.0, 800.0, 600.0),
           color: const Color(0xff000000),
         )
         ..line(
-          p1: const Offset(770.0, 10.0),
-          p2: const Offset(770.0, 590.0),
+          p1: const Offset(770.0, 00.0),
+          p2: const Offset(770.0, 600.0),
           strokeWidth: 1.0,
           color: const Color(0xffffeb3b),
         )

packages/flutter/test/widgets/scrollbar_test.dart

@@ -1779,7 +1779,7 @@ void main() {
     expect(
       find.byType(RawScrollbar),
       paints
-        ..rect(rect: const Rect.fromLTRB(794.0, 10.0, 800.0, 590.0))
+        ..rect(rect: const Rect.fromLTRB(794.0, 0.0, 800.0, 600.0))
         ..rect(rect: const Rect.fromLTRB(794.0, 10.0, 800.0, 358.0))
     );
   });