[framework] respect debugDisableShadows in slider thumbs (#105467)

Author: Jonah Williams

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

@@ -2366,7 +2366,19 @@ class RoundSliderThumbShape extends SliderComponentShape {
     final double evaluatedElevation = elevationTween.evaluate(activationAnimation);
     final Path path = Path()
       ..addArc(Rect.fromCenter(center: center, width: 2 * radius, height: 2 * radius), 0, math.pi * 2);
-    canvas.drawShadow(path, Colors.black, evaluatedElevation, true);
+
+    bool paintShadows = true;
+    assert(() {
+      if (debugDisableShadows) {
+        _debugDrawShadow(canvas, path, evaluatedElevation);
+        paintShadows = false;
+      }
+      return true;
+    }());
+
+    if (paintShadows) {
+      canvas.drawShadow(path, Colors.black, evaluatedElevation, true);
+    }
 
     canvas.drawCircle(
       center,
@@ -2475,7 +2487,19 @@ class RoundRangeSliderThumbShape extends RangeSliderThumbShape {
     final double evaluatedElevation = isPressed! ? elevationTween.evaluate(activationAnimation) : elevation;
     final Path shadowPath = Path()
       ..addArc(Rect.fromCenter(center: center, width: 2 * radius, height: 2 * radius), 0, math.pi * 2);
-    canvas.drawShadow(shadowPath, Colors.black, evaluatedElevation, true);
+
+    bool paintShadows = true;
+    assert(() {
+      if (debugDisableShadows) {
+        _debugDrawShadow(canvas, shadowPath, evaluatedElevation);
+        paintShadows = false;
+      }
+      return true;
+    }());
+
+    if (paintShadows) {
+      canvas.drawShadow(shadowPath, Colors.black, evaluatedElevation, true);
+    }
 
     canvas.drawCircle(
       center,
@@ -3359,3 +3383,15 @@ class RangeLabels {
     return '${objectRuntimeType(this, 'RangeLabels')}($start, $end)';
   }
 }
+
+void _debugDrawShadow(Canvas canvas, Path path, double elevation) {
+  if (elevation > 0.0) {
+    canvas.drawPath(
+      path,
+      Paint()
+        ..color = Colors.black
+        ..style = PaintingStyle.stroke
+        ..strokeWidth = elevation * 2.0,
+    );
+  }
+}

packages/flutter/test/material/range_slider_test.dart

@@ -1276,8 +1276,10 @@ void main() {
     expect(
       valueIndicatorBox,
       paints
+        ..path(color: Colors.black) // shadow
+        ..path(color: Colors.black) // shadow
         ..path(color: sliderTheme.valueIndicatorColor)
-        ..paragraph(),
+        ..paragraph()
     );
     await gesture.up();
     // Wait for value indicator animation to finish.
@@ -1360,7 +1362,7 @@ void main() {
     );
 
     // Represents the Raised Button and Range Slider.
-    expect(valueIndicatorBox, paintsExactlyCountTimes(#drawPath, 4));
+    expect(valueIndicatorBox, paintsExactlyCountTimes(#drawPath, 6));
     expect(valueIndicatorBox, paintsExactlyCountTimes(#drawParagraph, 3));
 
     await tester.tap(find.text('Next'));
@@ -1370,11 +1372,11 @@ void main() {
     expect(
       valueIndicatorBox,
       isNot(
-       paints
-         ..path(color: fillColor)
-         ..paragraph()
-         ..path(color: fillColor)
-         ..paragraph(),
+      paints
+        ..path(color: fillColor)
+        ..paragraph()
+        ..path(color: fillColor)
+        ..paragraph(),
       ),
     );
 
@@ -1519,6 +1521,8 @@ void main() {
     expect(
       valueIndicatorBox,
       paints
+        ..path(color: Colors.black) // shadow
+        ..path(color: Colors.black) // shadow
         ..path(color: sliderTheme.valueIndicatorColor)
         ..paragraph(),
     );
@@ -1594,6 +1598,8 @@ void main() {
     expect(
       valueIndicatorBox,
       paints
+        ..path(color: Colors.black) // shadow
+        ..path(color: Colors.black) // shadow
         ..path(color: sliderTheme.valueIndicatorColor)
         ..paragraph(),
     );