[Impeller] adjust coverage origin when rounding out SaveLayer bounds. (#161838)

Fixes https://github.com/flutter/flutter/issues/161374

When we round out the texture size for a saveLayer, also round out the
coverage so that the drawing origin is correct. Otherwise we may
actually cut off the top left edge of rendering.

---------

Co-authored-by: gaaclarke <30870216+gaaclarke@users.noreply.github.com>

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

@@ -1017,12 +1017,19 @@ void Canvas::SaveLayer(const Paint& paint,
   // sampling mode.
   ISize subpass_size;
   bool did_round_out = false;
+  Point coverage_origin_adjustment = Point{0, 0};
   if (paint.image_filter) {
     subpass_size = ISize(subpass_coverage.GetSize());
   } else {
     did_round_out = true;
     subpass_size =
         static_cast<ISize>(IRect::RoundOut(subpass_coverage).GetSize());
+    // If rounding out, adjust the coverage to account for the subpixel shift.
+    coverage_origin_adjustment =
+        Point(subpass_coverage.GetLeftTop().x -
+                  std::floor(subpass_coverage.GetLeftTop().x),
+              subpass_coverage.GetLeftTop().y -
+                  std::floor(subpass_coverage.GetLeftTop().y));
   }
   if (subpass_size.IsEmpty()) {
     return SkipUntilMatchingRestore(total_content_depth);
@@ -1156,7 +1163,8 @@ void Canvas::SaveLayer(const Paint& paint,
                                              subpass_size,              //
                                              Color::BlackTransparent()  //
                                              )));
-  save_layer_state_.push_back(SaveLayerState{paint_copy, subpass_coverage});
+  save_layer_state_.push_back(SaveLayerState{
+      paint_copy, subpass_coverage.Shift(-coverage_origin_adjustment)});
 
   CanvasStackEntry entry;
   entry.transform = transform_stack_.back().transform;

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

@@ -4,6 +4,9 @@
 
 #include "impeller/display_list/dl_golden_unittests.h"
 
+#include "display_list/dl_color.h"
+#include "display_list/dl_paint.h"
+#include "display_list/geometry/dl_geometry_types.h"
 #include "flutter/display_list/dl_builder.h"
 #include "flutter/impeller/geometry/path_builder.h"
 #include "flutter/testing/testing.h"
@@ -307,5 +310,30 @@ TEST_P(DlGoldenTest, DashedLinesTest) {
   ASSERT_TRUE(OpenPlaygroundHere(builder.Build()));
 }
 
+TEST_P(DlGoldenTest, SaveLayerAtFractionalValue) {
+  // Draws a stroked rounded rect at a fractional pixel value. The coverage must
+  // be adjusted so that we still have room to draw it, even though it lies on
+  // the fractional bounds of the saveLayer.
+  DisplayListBuilder builder;
+  builder.DrawPaint(DlPaint().setColor(DlColor::kWhite()));
+  auto save_paint = DlPaint().setAlpha(100);
+  builder.SaveLayer(nullptr, &save_paint);
+
+  builder.DrawRoundRect(DlRoundRect::MakeRectRadius(
+                            DlRect::MakeLTRB(10.5, 10.5, 200.5, 200.5), 10),
+                        DlPaint()
+                            .setDrawStyle(DlDrawStyle::kStroke)
+                            .setStrokeWidth(1.5)
+                            .setColor(DlColor::kBlack()));
+  builder.DrawCircle(DlPoint::MakeXY(100, 100), 50.5,
+                     DlPaint().setColor(DlColor::kAqua()));
+  builder.DrawCircle(DlPoint::MakeXY(110, 110), 50.5,
+                     DlPaint().setColor(DlColor::kCyan()));
+
+  builder.Restore();
+
+  ASSERT_TRUE(OpenPlaygroundHere(builder.Build()));
+}
+
 }  // namespace testing
 }  // namespace flutter

engine/src/flutter/testing/impeller_golden_tests_output.txt

@@ -981,6 +981,9 @@ impeller_Play_DlGoldenTest_GaussianVsRRectBlurScaled_Vulkan.png
 impeller_Play_DlGoldenTest_GaussianVsRRectBlur_Metal.png
 impeller_Play_DlGoldenTest_GaussianVsRRectBlur_OpenGLES.png
 impeller_Play_DlGoldenTest_GaussianVsRRectBlur_Vulkan.png
+impeller_Play_DlGoldenTest_SaveLayerAtFractionalValue_Metal.png
+impeller_Play_DlGoldenTest_SaveLayerAtFractionalValue_OpenGLES.png
+impeller_Play_DlGoldenTest_SaveLayerAtFractionalValue_Vulkan.png
 impeller_Play_DlGoldenTest_StrokedRRectFastBlur_Metal.png
 impeller_Play_DlGoldenTest_StrokedRRectFastBlur_OpenGLES.png
 impeller_Play_DlGoldenTest_StrokedRRectFastBlur_Vulkan.png