[display_list] allow applying opacity peephole to single glyph. (#53160)

A single glyph can be opacity peepholed, this is common in the case of Icons.

Author: Jonah Williams

display_list/BUILD.gn

@@ -131,6 +131,7 @@ if (enable_unittests) {
       ":display_list",
       ":display_list_fixtures",
       "//flutter/display_list/testing:display_list_testing",
+      "//flutter/impeller/typographer/backends/skia:typographer_skia_backend",
       "//flutter/testing",
       "//flutter/testing:skia",
     ]

display_list/display_list_unittests.cc

@@ -22,6 +22,7 @@
 #include "flutter/testing/display_list_testing.h"
 #include "flutter/testing/testing.h"
 
+#include "impeller/typographer/backends/skia/text_frame_skia.h"
 #include "third_party/skia/include/core/SkBBHFactory.h"
 #include "third_party/skia/include/core/SkColorFilter.h"
 #include "third_party/skia/include/core/SkPictureRecorder.h"
@@ -4331,5 +4332,30 @@ TEST_F(DisplayListTest, DrawDisplayListForwardsBackdropFlag) {
   EXPECT_TRUE(parent_dl->root_has_backdrop_filter());
 }
 
+TEST_F(DisplayListTest, TextFrameOpacityPeephole) {
+  // Single character can have opacity peephole applied.
+  {
+    std::string message = "A";
+    sk_sp<SkTextBlob> blob = CreateTextBlob(message);
+    auto frame = impeller::MakeTextFrameFromTextBlobSkia(blob);
+    DisplayListBuilder builder;
+    builder.DrawTextFrame(frame, 0, 0, {});
+    auto dl = builder.Build();
+    EXPECT_TRUE(dl->can_apply_group_opacity());
+  }
+
+  // Multiple characters cannot have opacity peephole applied.
+  {
+    std::string message = "ABC";
+    sk_sp<SkTextBlob> blob = CreateTextBlob(message);
+
+    auto frame = impeller::MakeTextFrameFromTextBlobSkia(blob);
+    DisplayListBuilder builder;
+    builder.DrawTextFrame(frame, 0, 0, {});
+    auto dl = builder.Build();
+    EXPECT_FALSE(dl->can_apply_group_opacity());
+  }
+}
+
 }  // namespace testing
 }  // namespace flutter

display_list/dl_builder.cc

@@ -1698,7 +1698,11 @@ void DisplayListBuilder::drawTextFrame(
     // they will protect overlapping glyphs from the effects of overdraw
     // so we must make the conservative assessment that this DL layer is
     // not compatible with group opacity inheritance.
-    UpdateLayerOpacityCompatibility(false);
+    // A single glyph can still have the opacity peephole applied (this is
+    // likely a glyph used as an Icon)
+    const bool is_single_glyph = text_frame->GetRunCount() == 1u &&
+                                 text_frame->GetRuns()[0].GetGlyphCount() == 1u;
+    UpdateLayerOpacityCompatibility(is_single_glyph);
     UpdateLayerResult(result);
   }
 }

display_list/testing/dl_test_snippets.cc

@@ -980,5 +980,12 @@ sk_sp<SkTextBlob> GetTestTextBlob(int index) {
   return blob;
 }
 
+sk_sp<SkTextBlob> CreateTextBlob(std::string& message) {
+  sk_sp<SkTextBlob> blob =
+      SkTextBlob::MakeFromText(message.c_str(), message.size(),
+                               CreateTestFontOfSize(20), SkTextEncoding::kUTF8);
+  return blob;
+}
+
 }  // namespace testing
 }  // namespace flutter

display_list/testing/dl_test_snippets.h

@@ -227,6 +227,8 @@ SkFont CreateTestFontOfSize(SkScalar scalar);
 
 sk_sp<SkTextBlob> GetTestTextBlob(int index);
 
+sk_sp<SkTextBlob> CreateTextBlob(std::string& message);
+
 struct DisplayListInvocation {
   // ----------------------------------
   // Required fields for initialization