Reset the state of the previous frame if overlay processing is skipped.

cc/output/overlay_processor.cc

Index: cc/output/overlay_processor.cc
diff --git a/cc/output/overlay_processor.cc b/cc/output/overlay_processor.cc
index 73e0a59a9b88dca34157e8f3e96ed3b99baca49d..e382c8ce58783ff62020f3c6fb80ee558deb3cd3 100644
--- a/cc/output/overlay_processor.cc
+++ b/cc/output/overlay_processor.cc
@@ -128,14 +128,17 @@ void OverlayProcessor::ProcessForOverlays(
   SendPromotionHintsBeforeReturning notifier(resource_provider, candidates);
 #endif
 
+  // If overlay processing is skipped, reset |previous_frame_underlay_rect_|.
+  // Otherwise, |previous_frame_underlay_rect_| will be set to current frame
+  // underlay rect.
+  const gfx::Rect previous_frame_underlay_rect = previous_frame_underlay_rect_;
+  previous_frame_underlay_rect_ = gfx::Rect();

[danakj, 2017/04/27 15:32:00]

Doesn't this just mean every frame previous_frame_underlay_rect_ = an empty
rect, so we never subtract damage?

The comment says "if overlay processing is skipped" but this is running when
ProcessFor*Layers() is called so, I'm not understanding what it means to say.

[AndyWu, 2017/04/27 15:53:01]

The thing we care about is UpdateDamageRect() being called or not. I am not
familiar with these terms. Maybe I should say "if hardware overlay processing is
skipped"? Or maybe I should directly say "if UpdateDamageRect() is not called"?

[danakj, 2017/04/27 16:26:19]

I see, UpdateDamageRect() is setting this to something not empty. OK Thanks.

+
   // If we have any copy requests, we can't remove any quads for overlays or
   // CALayers because the framebuffer would be missing the removed quads'
   // contents.
   if (!render_pass->copy_requests.empty()) {
     dc_processor_.ClearOverlayState();
-    // If overlay processing was skipped for a frame there's no way to be sure
-    // of the state of the previous frame, so reset.
-    previous_frame_underlay_rect_ = gfx::Rect();
     return;
   }
 
@@ -158,7 +161,7 @@ void OverlayProcessor::ProcessForOverlays(
                            content_bounds))
       continue;
 
-    UpdateDamageRect(candidates, damage_rect);
+    UpdateDamageRect(candidates, previous_frame_underlay_rect, damage_rect);
     return;
   }
 }
@@ -171,8 +174,10 @@ void OverlayProcessor::ProcessForOverlays(
 // not to swap the framebuffer there will still be a transparent hole in the
 // previous frame. This only handles the common case of a single underlay quad
 // for fullscreen video.
-void OverlayProcessor::UpdateDamageRect(OverlayCandidateList* candidates,
-                                        gfx::Rect* damage_rect) {
+void OverlayProcessor::UpdateDamageRect(
+    OverlayCandidateList* candidates,
+    const gfx::Rect& previous_frame_underlay_rect,
+    gfx::Rect* damage_rect) {
   gfx::Rect output_surface_overlay_damage_rect;
   gfx::Rect this_frame_underlay_rect;
   for (const OverlayCandidate& overlay : *candidates) {
@@ -189,7 +194,7 @@ void OverlayProcessor::UpdateDamageRect(OverlayCandidateList* candidates,
     }
   }
 
-  if (this_frame_underlay_rect == previous_frame_underlay_rect_)
+  if (this_frame_underlay_rect == previous_frame_underlay_rect)
     damage_rect->Subtract(this_frame_underlay_rect);
   previous_frame_underlay_rect_ = this_frame_underlay_rect;