Add `borderRadius` to LinearProgressIndicator (#123517)

Split from https://github.com/flutter/flutter/pull/122664 so it gets easier to review, as this is now unrelated to the `preferRound`. I'm one step from adding a width attribute, lol.

<img width="474" alt="image" src="https://user-images.githubusercontent.com/351125/226083798-71e529e9-4ae9-41de-a500-0412b989a273.png">

cc @Piinks

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

@@ -145,6 +145,7 @@ class _LinearProgressIndicatorPainter extends CustomPainter {
     this.value,
     required this.animationValue,
     required this.textDirection,
+    required this.indicatorBorderRadius,
   });
 
   final Color backgroundColor;
@@ -152,6 +153,7 @@ class _LinearProgressIndicatorPainter extends CustomPainter {
   final double? value;
   final double animationValue;
   final TextDirection textDirection;
+  final BorderRadiusGeometry indicatorBorderRadius;
 
   // The indeterminate progress animation displays two lines whose leading (head)
   // and trailing (tail) endpoints are defined by the following four curves.
@@ -181,7 +183,6 @@ class _LinearProgressIndicatorPainter extends CustomPainter {
     final Paint paint = Paint()
       ..color = backgroundColor
       ..style = PaintingStyle.fill;
-    canvas.drawRect(Offset.zero & size, paint);
 
     paint.color = valueColor;
 
@@ -197,7 +198,14 @@ class _LinearProgressIndicatorPainter extends CustomPainter {
         case TextDirection.ltr:
           left = x;
       }
-      canvas.drawRect(Offset(left, 0.0) & Size(width, size.height), paint);
+
+      final Rect rect = Offset(left, 0.0) & Size(width, size.height);
+      if (indicatorBorderRadius != BorderRadius.zero) {
+        final RRect rrect = indicatorBorderRadius.resolve(textDirection).toRRect(rect);
+        canvas.drawRRect(rrect, paint);
+      } else {
+        canvas.drawRect(rect, paint);
+      }
     }
 
     if (value != null) {
@@ -220,7 +228,8 @@ class _LinearProgressIndicatorPainter extends CustomPainter {
         || oldPainter.valueColor != valueColor
         || oldPainter.value != value
         || oldPainter.animationValue != animationValue
-        || oldPainter.textDirection != textDirection;
+        || oldPainter.textDirection != textDirection
+        || oldPainter.indicatorBorderRadius != indicatorBorderRadius;
   }
 }
 
@@ -279,6 +288,7 @@ class LinearProgressIndicator extends ProgressIndicator {
     this.minHeight,
     super.semanticsLabel,
     super.semanticsValue,
+    this.borderRadius = BorderRadius.zero,
   }) : assert(minHeight == null || minHeight > 0);
 
   /// {@template flutter.material.LinearProgressIndicator.trackColor}
@@ -301,6 +311,12 @@ class LinearProgressIndicator extends ProgressIndicator {
   /// {@endtemplate}
   final double? minHeight;
 
+  /// The border radius of both the indicator and the track.
+  ///
+  /// By default it is [BorderRadius.zero], which produces a rectangular shape
+  /// with a rectangular indicator.
+  final BorderRadiusGeometry borderRadius;
+
   @override
   State<LinearProgressIndicator> createState() => _LinearProgressIndicatorState();
 }
@@ -352,6 +368,14 @@ class _LinearProgressIndicatorState extends State<LinearProgressIndicator> with
     return widget._buildSemanticsWrapper(
       context: context,
       child: Container(
+        // Clip is only needed with indeterminate progress indicators
+        clipBehavior: (widget.borderRadius != BorderRadius.zero && widget.value == null)
+            ? Clip.antiAlias
+            : Clip.none,
+        decoration: ShapeDecoration(
+          color: trackColor,
+          shape: RoundedRectangleBorder(borderRadius: widget.borderRadius),
+        ),
         constraints: BoxConstraints(
           minWidth: double.infinity,
           minHeight: minHeight,
@@ -363,6 +387,7 @@ class _LinearProgressIndicatorState extends State<LinearProgressIndicator> with
             value: widget.value, // may be null
             animationValue: animationValue, // ignored if widget.value is not null
             textDirection: textDirection,
+            indicatorBorderRadius: widget.borderRadius,
           ),
         ),
       ),

packages/flutter/test/material/progress_indicator_test.dart

@@ -465,6 +465,41 @@ void main() {
     expect(find.byType(CircularProgressIndicator), paints..arc(strokeCap: StrokeCap.round));
   });
 
+  testWidgets('LinearProgressIndicator with indicatorBorderRadius', (WidgetTester tester) async {
+    await tester.pumpWidget(
+      Theme(
+        data: theme,
+        child: const Directionality(
+          textDirection: TextDirection.ltr,
+          child: Center(
+            child: SizedBox(
+              width: 100.0,
+              height: 4.0,
+              child: LinearProgressIndicator(
+                value: 0.25,
+                borderRadius: BorderRadius.all(Radius.circular(10)),
+              ),
+            ),
+          ),
+        ),
+      ),
+    );
+    expect(
+        find.byType(LinearProgressIndicator),
+        paints
+        ..rrect(
+          rrect: RRect.fromLTRBR(0.0, 0.0, 100.0, 4.0, const Radius.circular(10.0)),
+        )
+        ..rrect(
+          rrect: RRect.fromRectAndRadius(
+            const Rect.fromLTRB(0.0, 0.0, 25.0, 4.0),
+            const Radius.circular(10.0),
+          ),
+        ),
+    );
+    expect(tester.binding.transientCallbackCount, 0);
+  });
+
   testWidgets('CircularProgressIndicator paint colors', (WidgetTester tester) async {
     const Color green = Color(0xFF00FF00);
     const Color blue = Color(0xFF0000FF);