fix(scrollbar): Update padding type to EdgeInsetsGeometry (flutter#172056)

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This PR enhances RTL support for the RawScrollbar widget by updating its
padding type and adding comprehensive tests.

The changes include:
1. Updating RawScrollbar's `padding` parameter from `EdgeInsets` to
`EdgeInsetsGeometry`
- This change is non-breaking because `EdgeInsets` is a subclass of
`EdgeInsetsGeometry`
   - Existing code using `EdgeInsets` will continue to work as before
- Users can now optionally use `EdgeInsetsDirectional` for RTL support
2. Adding a test case that verifies scrollbar positioning with
EdgeInsetsDirectional padding
3. Testing both overscroll and regular scroll scenarios with directional
padding

These changes ensure that the scrollbar works correctly in both LTR and
RTL layouts, improving accessibility and internationalization support.

Fixes: flutter#167922

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Author: SalehTZ

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

@@ -64,6 +64,9 @@ enum ScrollbarOrientation {
 /// proportional to the percentage of content completely visible on screen,
 /// as long as its size isn't less than [minLength] and it isn't overscrolling.
 ///
+/// If [padding] is an [EdgeInsetsDirectional], a non-null [textDirection] must
+/// be provided to properly resolve the padding values.
+///
 /// Unlike [CustomPainter]s that subclasses [CustomPainter] and only repaint
 /// when [shouldRepaint] returns true (which requires this [CustomPainter] to
 /// be rebuilt), this painter has the added optimization of repainting and not
@@ -95,7 +98,7 @@ class ScrollbarPainter extends ChangeNotifier implements CustomPainter {
     Color trackBorderColor = const Color(0x00000000),
     TextDirection? textDirection,
     double thickness = _kScrollbarThickness,
-    EdgeInsets padding = EdgeInsets.zero,
+    EdgeInsetsGeometry padding = EdgeInsets.zero,
     double mainAxisMargin = 0.0,
     double crossAxisMargin = 0.0,
     Radius? radius,
@@ -110,12 +113,17 @@ class ScrollbarPainter extends ChangeNotifier implements CustomPainter {
        assert(minOverscrollLength == null || minOverscrollLength <= minLength),
        assert(minOverscrollLength == null || minOverscrollLength >= 0),
        assert(padding.isNonNegative),
+       assert(
+         padding is! EdgeInsetsDirectional || textDirection != null,
+         'A non-null textDirection must be provided when using EdgeInsetsDirectional for padding.',
+       ),
        _color = color,
        _textDirection = textDirection,
        _thickness = thickness,
        _radius = radius,
        _shape = shape,
        _padding = padding,
+       _resolvedPadding = padding.resolve(textDirection),
        _mainAxisMargin = mainAxisMargin,
        _crossAxisMargin = crossAxisMargin,
        _minLength = minLength,
@@ -190,6 +198,7 @@ class ScrollbarPainter extends ChangeNotifier implements CustomPainter {
     }
 
     _textDirection = value;
+    _resolvedPadding = _padding.resolve(_textDirection);
     notifyListeners();
   }
 
@@ -290,14 +299,20 @@ class ScrollbarPainter extends ChangeNotifier implements CustomPainter {
   ///
   /// Defaults to [EdgeInsets.zero]. Offsets from all four directions must be
   /// greater than or equal to zero.
-  EdgeInsets get padding => _padding;
-  EdgeInsets _padding;
-  set padding(EdgeInsets value) {
+  ///
+  /// For RTL (right-to-left) support, you can provide [EdgeInsetsDirectional],
+  /// but you must also provide a non-null [textDirection] to properly resolve
+  /// the padding values. The scrollbar will automatically adjust the padding
+  /// based on the text direction.
+  EdgeInsetsGeometry get padding => _padding;
+  EdgeInsetsGeometry _padding;
+  set padding(EdgeInsetsGeometry value) {
     if (padding == value) {
       return;
     }
 
     _padding = value;
+    _resolvedPadding = _padding.resolve(_textDirection);
     notifyListeners();
   }
 
@@ -388,17 +403,19 @@ class ScrollbarPainter extends ChangeNotifier implements CustomPainter {
   // - Scrollbar Details
 
   Rect? _trackRect;
+  EdgeInsets? _resolvedPadding;
   // The full painted length of the track
   double get _trackExtent => _lastMetrics!.viewportDimension - _totalTrackMainAxisOffsets;
   // The full length of the track that the thumb can travel
   double get _traversableTrackExtent => _trackExtent - (2 * mainAxisMargin);
   // Track Offsets
   // The track is offset by only padding.
-  double get _totalTrackMainAxisOffsets => _isVertical ? padding.vertical : padding.horizontal;
+  double get _totalTrackMainAxisOffsets =>
+      _isVertical ? _resolvedPadding!.vertical : _resolvedPadding!.horizontal;
 
   double get _leadingTrackMainAxisOffset => switch (_resolvedOrientation) {
-    ScrollbarOrientation.left || ScrollbarOrientation.right => padding.top,
-    ScrollbarOrientation.top || ScrollbarOrientation.bottom => padding.left,
+    ScrollbarOrientation.left || ScrollbarOrientation.right => _resolvedPadding!.top,
+    ScrollbarOrientation.top || ScrollbarOrientation.bottom => _resolvedPadding!.left,
   };
 
   Rect? _thumbRect;
@@ -557,7 +574,6 @@ class ScrollbarPainter extends ChangeNotifier implements CustomPainter {
       textDirection != null,
       'A TextDirection must be provided before a Scrollbar can be painted.',
     );
-
     final double x, y;
     final Size thumbSize, trackSize;
     final Offset trackOffset, borderStart, borderEnd;
@@ -566,15 +582,15 @@ class ScrollbarPainter extends ChangeNotifier implements CustomPainter {
       case ScrollbarOrientation.left:
         thumbSize = Size(thickness, _thumbExtent);
         trackSize = Size(thickness + 2 * crossAxisMargin, _trackExtent);
-        x = crossAxisMargin + padding.left;
+        x = crossAxisMargin + _resolvedPadding!.left;
         y = _thumbOffset;
         trackOffset = Offset(x - crossAxisMargin, _leadingTrackMainAxisOffset);
         borderStart = trackOffset + Offset(trackSize.width, 0.0);
         borderEnd = Offset(trackOffset.dx + trackSize.width, trackOffset.dy + _trackExtent);
       case ScrollbarOrientation.right:
         thumbSize = Size(thickness, _thumbExtent);
         trackSize = Size(thickness + 2 * crossAxisMargin, _trackExtent);
-        x = size.width - thickness - crossAxisMargin - padding.right;
+        x = size.width - thickness - crossAxisMargin - _resolvedPadding!.right;
         y = _thumbOffset;
         trackOffset = Offset(x - crossAxisMargin, _leadingTrackMainAxisOffset);
         borderStart = trackOffset;
@@ -583,15 +599,15 @@ class ScrollbarPainter extends ChangeNotifier implements CustomPainter {
         thumbSize = Size(_thumbExtent, thickness);
         trackSize = Size(_trackExtent, thickness + 2 * crossAxisMargin);
         x = _thumbOffset;
-        y = crossAxisMargin + padding.top;
+        y = crossAxisMargin + _resolvedPadding!.top;
         trackOffset = Offset(_leadingTrackMainAxisOffset, y - crossAxisMargin);
         borderStart = trackOffset + Offset(0.0, trackSize.height);
         borderEnd = Offset(trackOffset.dx + _trackExtent, trackOffset.dy + trackSize.height);
       case ScrollbarOrientation.bottom:
         thumbSize = Size(_thumbExtent, thickness);
         trackSize = Size(_trackExtent, thickness + 2 * crossAxisMargin);
         x = _thumbOffset;
-        y = size.height - thickness - crossAxisMargin - padding.bottom;
+        y = size.height - thickness - crossAxisMargin - _resolvedPadding!.bottom;
         trackOffset = Offset(_leadingTrackMainAxisOffset, y - crossAxisMargin);
         borderStart = trackOffset;
         borderEnd = Offset(trackOffset.dx + _trackExtent, trackOffset.dy);
@@ -634,6 +650,7 @@ class ScrollbarPainter extends ChangeNotifier implements CustomPainter {
     if (_lastAxisDirection == null || !_needPaint(_lastMetrics)) {
       return;
     }
+
     // Skip painting if there's not enough space.
     if (_traversableTrackExtent <= 0) {
       return;
@@ -1320,7 +1337,7 @@ class RawScrollbar extends StatefulWidget {
   /// When null, the inherited [MediaQueryData.padding] is used.
   ///
   /// Defaults to null.
-  final EdgeInsets? padding;
+  final EdgeInsetsGeometry? padding;
 
   @override
   RawScrollbarState<RawScrollbar> createState() => RawScrollbarState<RawScrollbar>();
@@ -1543,6 +1560,7 @@ class RawScrollbarState<T extends RawScrollbar> extends State<T> with TickerProv
   /// Subclasses can override to configure the [scrollbarPainter].
   @protected
   void updateScrollbarPainter() {
+    final TextDirection textDirection = Directionality.of(context);
     scrollbarPainter
       ..color = widget.thumbColor ?? const Color(0x66BCBCBC)
       ..trackRadius = widget.trackRadius
@@ -1552,10 +1570,10 @@ class RawScrollbarState<T extends RawScrollbar> extends State<T> with TickerProv
       ..trackBorderColor = _showTrack
           ? widget.trackBorderColor ?? const Color(0x1a000000)
           : const Color(0x00000000)
-      ..textDirection = Directionality.of(context)
+      ..textDirection = textDirection
       ..thickness = widget.thickness ?? _kScrollbarThickness
       ..radius = widget.radius
-      ..padding = widget.padding ?? MediaQuery.paddingOf(context)
+      ..padding = (widget.padding ?? MediaQuery.paddingOf(context)).resolve(textDirection)
       ..scrollbarOrientation = widget.scrollbarOrientation
       ..mainAxisMargin = widget.mainAxisMargin
       ..shape = widget.shape

packages/flutter/test/widgets/scrollbar_test.dart

@@ -17,7 +17,7 @@ const Duration _kScrollbarTimeToFade = Duration(milliseconds: 600);
 
 ScrollbarPainter _buildPainter({
   TextDirection textDirection = TextDirection.ltr,
-  EdgeInsets padding = EdgeInsets.zero,
+  EdgeInsetsGeometry padding = EdgeInsets.zero,
   Color color = _kScrollbarColor,
   double thickness = _kThickness,
   double mainAxisMargin = 0.0,
@@ -3567,4 +3567,68 @@ The provided ScrollController cannot be shared by multiple ScrollView widgets.''
     },
     variant: TargetPlatformVariant.desktop(),
   );
+
+  test('with EdgeInsetsDirectional', () {
+    const Size size = Size(60, 80);
+    final ScrollMetrics metrics = defaultMetrics.copyWith(
+      minScrollExtent: -100,
+      maxScrollExtent: 240,
+      axisDirection: AxisDirection.down,
+    );
+
+    final ScrollbarPainter ltrPainter = _buildPainter(
+      padding: const EdgeInsetsDirectional.fromSTEB(1, 2, 3, 4),
+      scrollMetrics: metrics,
+    );
+
+    ltrPainter.update(
+      metrics.copyWith(viewportDimension: size.height, pixels: double.negativeInfinity),
+      AxisDirection.down,
+    );
+
+    // Top overscroll.
+    ltrPainter.paint(testCanvas, size);
+    final Rect ltrRect0 = captureRect();
+    expect(ltrRect0.top, 2);
+    expect(size.width - ltrRect0.right, 3);
+
+    // Bottom overscroll.
+    ltrPainter.update(
+      metrics.copyWith(viewportDimension: size.height, pixels: double.infinity),
+      AxisDirection.down,
+    );
+
+    ltrPainter.paint(testCanvas, size);
+    final Rect ltrRect1 = captureRect();
+    expect(size.height - ltrRect1.bottom, 4);
+    expect(size.width - ltrRect1.right, 3);
+
+    final ScrollbarPainter rtlPainter = _buildPainter(
+      padding: const EdgeInsetsDirectional.fromSTEB(1, 2, 3, 4),
+      scrollMetrics: metrics,
+      textDirection: TextDirection.rtl,
+    );
+
+    rtlPainter.update(
+      metrics.copyWith(viewportDimension: size.height, pixels: double.negativeInfinity),
+      AxisDirection.down,
+    );
+
+    // Top overscroll.
+    rtlPainter.paint(testCanvas, size);
+    final Rect rtlRect0 = captureRect();
+    expect(rtlRect0.top, 2);
+    expect(rtlRect0.left, 3);
+
+    // Bottom overscroll.
+    rtlPainter.update(
+      metrics.copyWith(viewportDimension: size.height, pixels: double.infinity),
+      AxisDirection.down,
+    );
+
+    rtlPainter.paint(testCanvas, size);
+    final Rect rtlRect1 = captureRect();
+    expect(size.height - rtlRect1.bottom, 4);
+    expect(rtlRect1.left, 3);
+  });
 }