cc: Don't subtract transparent overlay from damage.

cc/output/overlay_processor.cc

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "cc/output/overlay_processor.h"
 
 #include "cc/output/dc_layer_overlay.h"
 #include "cc/output/output_surface.h"
 #include "cc/output/overlay_strategy_single_on_top.h"
 #include "cc/output/overlay_strategy_underlay.h"
@@ skipping 142 matching lines @@
   for (const auto& strategy : strategies_) {
     if (!strategy->Attempt(resource_provider, render_pass, candidates,
                            content_bounds))
       continue;
 
     UpdateDamageRect(candidates, damage_rect);
     return;
   }
 }
 
-// Subtract on-top overlays from the damage rect, unless the overlays use
+// Subtract on-top opaque overlays from the damage rect, unless the overlays use
 // the backbuffer as their content (in which case, add their combined rect
 // back to the damage at the end).
 // Also subtract unoccluded underlays from the damage rect if we know that the
 // same underlay was scheduled on the previous frame. If the renderer decides
 // 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) {
   gfx::Rect output_surface_overlay_damage_rect;
   gfx::Rect this_frame_underlay_rect;
   for (const OverlayCandidate& overlay : *candidates) {
-    if (overlay.plane_z_order > 0) {
+    if (overlay.plane_z_order > 0 && overlay.is_opaque) {
       const gfx::Rect overlay_display_rect =
           ToEnclosedRect(overlay.display_rect);
       overlay_damage_rect_.Union(overlay_display_rect);
       damage_rect->Subtract(overlay_display_rect);
       if (overlay.use_output_surface_for_resource)
         output_surface_overlay_damage_rect.Union(overlay_display_rect);
     } else if (overlay.plane_z_order < 0 && overlay.is_unoccluded &&
                this_frame_underlay_rect.IsEmpty()) {
       this_frame_underlay_rect = ToEnclosedRect(overlay.display_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;
 
   damage_rect->Union(output_surface_overlay_damage_rect);
 }
 
 }  // namespace cc