Fix damage calculation when not providing populate_existing_damage fo…

…r gl embedder (#45611)

# Description

This PR fixes the `gl_populate_existing_damage` in embedder.cc, which currently returns an empty rectangle when a full repaint is needed. This leads to the `frame_damage` being considered empty in rasterizer.cc, causing incorrect partial repaint within Flutter when `gl_populate_existing_damage` is not provided by the embedder.

# Related Issue

flutter/flutter#119601

# Tests

Add a new test. Also fixes damage calculation related tests in EmbedderTest by

* Use a new Dart embedder fixture entry point `render_gradient_retained` which retains the old layer to make sure layer tree diff happens.
* Add `latch.Wait()` after `SetGLPresentCallback` to make sure assertions have been executed.
* Make `existing_damage_rects` static since it should be valid after `populate_existing_damage` returns.

shell/gpu/gpu_surface_gl_delegate.h

@@ -27,8 +27,6 @@ struct GLFrameInfo {
 struct GLFBOInfo {
   // The frame buffer's ID.
   uint32_t fbo_id;
-  // This boolean flags whether the returned FBO supports partial repaint.
-  const bool partial_repaint_enabled;
   // The frame buffer's existing damage (i.e. damage since it was last used).
   const std::optional<SkIRect> existing_damage;
 };

shell/platform/android/android_surface_gl_skia.cc

@@ -166,7 +166,6 @@ GLFBOInfo AndroidSurfaceGLSkia::GLContextFBO(GLFrameInfo frame_info) const {
   // The default window bound framebuffer on Android.
   return GLFBOInfo{
       .fbo_id = 0,
-      .partial_repaint_enabled = onscreen_surface_->SupportsPartialRepaint(),
       .existing_damage = onscreen_surface_->InitialDamage(),
   };
 }

shell/platform/embedder/embedder.cc

@@ -363,38 +363,30 @@ InferOpenGLPlatformViewCreationCallback(
     if (!populate_existing_damage) {
       return flutter::GLFBOInfo{
           .fbo_id = static_cast<uint32_t>(id),
-          .partial_repaint_enabled = false,
-          .existing_damage = SkIRect::MakeEmpty(),
+          .existing_damage = std::nullopt,
       };
     }
 
     // Given the FBO's ID, get its existing damage.
     FlutterDamage existing_damage;
     populate_existing_damage(user_data, id, &existing_damage);
 
-    bool partial_repaint_enabled = true;
-    SkIRect existing_damage_rect;
+    std::optional<SkIRect> existing_damage_rect = std::nullopt;
 
     // Verify that at least one damage rectangle was provided.
     if (existing_damage.num_rects <= 0 || existing_damage.damage == nullptr) {
       FML_LOG(INFO) << "No damage was provided. Forcing full repaint.";
-      existing_damage_rect = SkIRect::MakeEmpty();
-      partial_repaint_enabled = false;
-    } else if (existing_damage.num_rects > 1) {
-      // Log message notifying users that multi-damage is not yet available in
-      // case they try to make use of it.
-      FML_LOG(INFO) << "Damage with multiple rectangles not yet supported. "
-                       "Repainting the whole frame.";
-      existing_damage_rect = SkIRect::MakeEmpty();
-      partial_repaint_enabled = false;
     } else {
-      existing_damage_rect = FlutterRectToSkIRect(*(existing_damage.damage));
+      existing_damage_rect = SkIRect::MakeEmpty();
+      for (size_t i = 0; i < existing_damage.num_rects; i++) {
+        existing_damage_rect->join(
+            FlutterRectToSkIRect(existing_damage.damage[i]));
+      }
     }
 
     // Pass the information about this FBO to the rendering backend.
     return flutter::GLFBOInfo{
         .fbo_id = static_cast<uint32_t>(id),
-        .partial_repaint_enabled = partial_repaint_enabled,
         .existing_damage = existing_damage_rect,
     };
   };

shell/platform/embedder/fixtures/main.dart

@@ -1298,3 +1298,25 @@ void channel_listener_response() async {
   });
   signalNativeTest();
 }
+
+@pragma('vm:entry-point')
+void render_gradient_retained() {
+  OffsetEngineLayer? offsetLayer; // Retain the offset layer.
+  PlatformDispatcher.instance.onBeginFrame = (Duration duration) {
+    Size size = Size(800.0, 600.0);
+
+    SceneBuilder builder = SceneBuilder();
+
+    offsetLayer = builder.pushOffset(0.0, 0.0, oldLayer: offsetLayer);
+
+    // display_list_layer will comparing the display_list
+    // no need to retain the picture
+    builder.addPicture(
+        Offset(0.0, 0.0), CreateGradientBox(size)); // gradient - flutter
+
+    builder.pop();
+
+    PlatformDispatcher.instance.views.first.render(builder.build());
+  };
+  PlatformDispatcher.instance.scheduleFrame();
+}