[Impeller] Move to the new location before rendering a stroke path contour containing only one point (#165940)

Fixes flutter/flutter#165190

Author: jason-simmons

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

@@ -641,5 +641,25 @@ TEST_P(AiksTest, CanRenderOverlappingMultiContourPath) {
   ASSERT_TRUE(OpenPlaygroundHere(builder.Build()));
 }
 
+TEST_P(AiksTest, TwoContourPathWithSinglePointContour) {
+  DisplayListBuilder builder;
+
+  DlPaint paint;
+  paint.setColor(DlColor::kRed());
+  paint.setDrawStyle(DlDrawStyle::kStroke);
+  paint.setStrokeWidth(15.0);
+  paint.setStrokeCap(DlStrokeCap::kRound);
+
+  DlPathBuilder path_builder;
+  path_builder.MoveTo(DlPoint(100, 100));
+  path_builder.LineTo(DlPoint(150, 150));
+  path_builder.MoveTo(DlPoint(200, 200));
+  path_builder.LineTo(DlPoint(200, 200));
+
+  builder.DrawPath(DlPath(path_builder), paint);
+
+  ASSERT_TRUE(OpenPlaygroundHere(builder.Build()));
+}
+
 }  // namespace testing
 }  // namespace impeller

engine/src/flutter/impeller/entity/geometry/stroke_path_geometry.cc

@@ -87,22 +87,10 @@ class StrokeGenerator {
           polyline.GetContourPointBounds(contour_i);
 
       size_t contour_delta = contour_end_point_i - contour_start_point_i;
-      if (contour_delta == 1) {
-        Point p = polyline.GetPoint(contour_start_point_i);
-        cap_proc(vtx_builder, p, {-stroke_width * 0.5f, 0}, scale,
-                 /*reverse=*/false);
-        cap_proc(vtx_builder, p, {stroke_width * 0.5f, 0}, scale,
-                 /*reverse=*/false);
-        continue;
-      } else if (contour_delta == 0) {
+      if (contour_delta == 0) {
         continue;  // This contour has no renderable content.
       }
 
-      previous_offset = offset;
-      offset = ComputeOffset(contour_start_point_i, contour_start_point_i,
-                             contour_end_point_i, contour);
-      const Point contour_first_offset = offset.GetVector();
-
       if (contour_i > 0) {
         // This branch only executes when we've just finished drawing a contour
         // and are switching to a new one.
@@ -125,6 +113,20 @@ class StrokeGenerator {
         vtx_builder.AppendVertex(vtx.position);
       }
 
+      if (contour_delta == 1) {
+        Point p = polyline.GetPoint(contour_start_point_i);
+        cap_proc(vtx_builder, p, {-stroke_width * 0.5f, 0}, scale,
+                 /*reverse=*/false);
+        cap_proc(vtx_builder, p, {stroke_width * 0.5f, 0}, scale,
+                 /*reverse=*/false);
+        continue;
+      }
+
+      previous_offset = offset;
+      offset = ComputeOffset(contour_start_point_i, contour_start_point_i,
+                             contour_end_point_i, contour);
+      const Point contour_first_offset = offset.GetVector();
+
       // Generate start cap.
       if (!polyline.contours[contour_i].is_closed) {
         Point cap_offset =

engine/src/flutter/testing/impeller_golden_tests_output.txt

@@ -963,6 +963,9 @@ impeller_Play_AiksTest_TranslucentSaveLayerWithColorMatrixImageFilterDrawsCorrec
 impeller_Play_AiksTest_TransparentShadowProducesCorrectColor_Metal.png
 impeller_Play_AiksTest_TransparentShadowProducesCorrectColor_OpenGLES.png
 impeller_Play_AiksTest_TransparentShadowProducesCorrectColor_Vulkan.png
+impeller_Play_AiksTest_TwoContourPathWithSinglePointContour_Metal.png
+impeller_Play_AiksTest_TwoContourPathWithSinglePointContour_OpenGLES.png
+impeller_Play_AiksTest_TwoContourPathWithSinglePointContour_Vulkan.png
 impeller_Play_AiksTest_VerifyNonOptimizedGradient_Metal.png
 impeller_Play_AiksTest_VerifyNonOptimizedGradient_OpenGLES.png
 impeller_Play_AiksTest_VerifyNonOptimizedGradient_Vulkan.png