Added special case for fat width arcs (#161255)

fixes https://github.com/flutter/flutter/issues/158567

This draws arcs as oval sectors when the stroke width is large enough.

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above.
- [x] I updated/added relevant documentation (doc comments with `///`).
- [x] I added new tests to check the change I am making, or this PR is
[test-exempt].
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Fixes] where supported.
- [x] All existing and new tests are passing.

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engine/src/flutter/impeller/display_list/aiks_dl_path_unittests.cc

@@ -461,6 +461,45 @@ TEST_P(AiksTest, CanRenderClips) {
   ASSERT_TRUE(OpenPlaygroundHere(builder.Build()));
 }
 
+TEST_P(AiksTest, FatStrokeArc) {
+  DlScalar stroke_width = 300;
+  DlScalar aspect = 1.0;
+  DlScalar start_angle = 0;
+  DlScalar end_angle = 90;
+  auto callback = [&]() -> sk_sp<DisplayList> {
+    if (AiksTest::ImGuiBegin("Controls", nullptr,
+                             ImGuiWindowFlags_AlwaysAutoResize)) {
+      ImGui::SliderFloat("Stroke Width", &stroke_width, 1, 300);
+      ImGui::SliderFloat("Aspect", &aspect, 0.5, 2.0);
+      ImGui::SliderFloat("Start Angle", &start_angle, 0, 360);
+      ImGui::SliderFloat("End Angle", &end_angle, 0, 360);
+      ImGui::End();
+    }
+
+    DisplayListBuilder builder;
+    DlPaint grey_paint;
+    grey_paint.setColor(DlColor(0xff111111));
+    builder.DrawPaint(grey_paint);
+
+    DlPaint white_paint;
+    white_paint.setColor(DlColor::kWhite());
+    white_paint.setStrokeWidth(stroke_width);
+    white_paint.setDrawStyle(DlDrawStyle::kStroke);
+    DlPaint red_paint;
+    red_paint.setColor(DlColor::kRed());
+
+    Rect rect = Rect::MakeXYWH(100, 100, 100, aspect * 100);
+    builder.DrawRect(rect, red_paint);
+    builder.DrawArc(rect, start_angle, end_angle,
+                    /*useCenter=*/false, white_paint);
+    DlScalar frontier = rect.GetRight() + stroke_width / 2.0;
+    builder.DrawLine(Point(frontier, 0), Point(frontier, 150), red_paint);
+
+    return builder.Build();
+  };
+  ASSERT_TRUE(OpenPlaygroundHere(callback));
+}
+
 TEST_P(AiksTest, CanRenderOverlappingMultiContourPath) {
   DisplayListBuilder builder;
 

engine/src/flutter/impeller/display_list/dl_dispatcher.cc

@@ -642,10 +642,26 @@ void DlDispatcherBase::drawArc(const DlRect& oval_bounds,
                                bool use_center) {
   AUTO_DEPTH_WATCHER(1u);
 
-  PathBuilder builder;
-  builder.AddArc(oval_bounds, Degrees(start_degrees), Degrees(sweep_degrees),
-                 use_center);
-  GetCanvas().DrawPath(builder.TakePath(), paint_);
+  if (paint_.stroke_width >
+      std::max(oval_bounds.GetWidth(), oval_bounds.GetHeight())) {
+    // This is a special case for rendering arcs whose stroke width is so large
+    // you are effectively drawing a sector of a circle.
+    // https://github.com/flutter/flutter/issues/158567
+    DlRect expanded_rect = oval_bounds.Expand(Size(paint_.stroke_width / 2));
+    PathBuilder builder;
+    Paint fill_paint = paint_;
+    fill_paint.style = Paint::Style::kFill;
+    fill_paint.stroke_width = 1;
+    builder.AddArc(expanded_rect, Degrees(start_degrees),
+                   Degrees(sweep_degrees),
+                   /*use_center=*/true);
+    GetCanvas().DrawPath(builder.TakePath(), fill_paint);
+  } else {
+    PathBuilder builder;
+    builder.AddArc(oval_bounds, Degrees(start_degrees), Degrees(sweep_degrees),
+                   use_center);
+    GetCanvas().DrawPath(builder.TakePath(), paint_);
+  }
 }
 
 // |flutter::DlOpReceiver|

engine/src/flutter/testing/impeller_golden_tests_output.txt

@@ -690,6 +690,9 @@ impeller_Play_AiksTest_FastGradientTestVerticalReversed_Vulkan.png
 impeller_Play_AiksTest_FastGradientTestVertical_Metal.png
 impeller_Play_AiksTest_FastGradientTestVertical_OpenGLES.png
 impeller_Play_AiksTest_FastGradientTestVertical_Vulkan.png
+impeller_Play_AiksTest_FatStrokeArc_Metal.png
+impeller_Play_AiksTest_FatStrokeArc_OpenGLES.png
+impeller_Play_AiksTest_FatStrokeArc_Vulkan.png
 impeller_Play_AiksTest_FilledCirclesRenderCorrectly_Metal.png
 impeller_Play_AiksTest_FilledCirclesRenderCorrectly_OpenGLES.png
 impeller_Play_AiksTest_FilledCirclesRenderCorrectly_Vulkan.png