Underlays on macOS: Add TODO for new strategy and do some renames

Add comments clarify the reason that the DrawQuadList is not cleared
by the CALayer overlay processor (the RenderPassDrawQuads are sill
referenced).

Add a TODO for the place to add the underlay processor.

Bug: 1152849
Change-Id: Iaf58b2c4ebbadf29c40ed76660069f9d0932a995
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2587949
Commit-Queue: ccameron <ccameron@chromium.org>
Reviewed-by: Chris Blume <cblume@chromium.org>
Cr-Commit-Position: refs/heads/master@{#839808}

components/viz/service/display/overlay_processor_mac.cc

@@ -58,7 +58,6 @@ void OverlayProcessorMac::ProcessForOverlays(
 
   // Clear to get ready to handle output surface as overlay.
   output_surface_already_handled_ = false;
-  previous_frame_full_bounding_rect_ = render_pass->output_rect;
 
   // Skip overlay processing if we have copy request.
   if (!render_pass->copy_requests.empty())
@@ -69,19 +68,30 @@ void OverlayProcessorMac::ProcessForOverlays(
   if (!enable_ca_overlay_)
     return;
 
-  // If ca overlay system didn't succeed, we fall back to surfaceless.
-  if (!ca_layer_overlay_processor_->ProcessForCALayerOverlays(
+  // First, try to use ProcessForCALayerOverlays to replace all DrawQuads in
+  // |render_pass->quad_list| with CALayerOverlays in |candidates|.
+  if (ca_layer_overlay_processor_->ProcessForCALayerOverlays(
           resource_provider, gfx::RectF(render_pass->output_rect),
           render_pass->quad_list, render_pass_filters,
-          render_pass_backdrop_filters, candidates))
-    return;
-
-  // CALayer overlays are all-or-nothing. If all quads were replaced with
-  // layers then mark the output surface as already handled.
+          render_pass_backdrop_filters, candidates)) {
+    // Mark the output surface as already handled (there is no output surface
+    // anymore).
     output_surface_already_handled_ = true;
+
+    // Set |last_overlay_damage_| to be everything, so that the next
+    // frame that we draw to the output surface will do a full re-draw.
     ca_overlay_damage_rect_ = render_pass->output_rect;
-  previous_frame_full_bounding_rect_ = ca_overlay_damage_rect_;
+
+    // Everything in |render_pass->quad_list| has been moved over to
+    // |candidates|. Ideally we would clear |render_pass->quad_list|, but some
+    // RenderPass overlays still point into that list. So instead, to avoid
+    // drawing the root RenderPass, we set |damage_rect| to be empty.
     *damage_rect = gfx::Rect();
+  }
+
+  // TODO(https://crbug.com/1152849): If there is any HDR or protected content,
+  // use an underlay strategy to move those to |candidates| and replace them
+  // with transparent quads in |render_pass->quad_list|.
 }
 
 void OverlayProcessorMac::AdjustOutputSurfaceOverlay(

components/viz/service/display/overlay_processor_mac.h

@@ -67,8 +67,10 @@ class VIZ_SERVICE_EXPORT OverlayProcessorMac
 
  private:
   const bool enable_ca_overlay_;
+
+  // The damage that should be added the next frame for drawing to the output
+  // surface.
   gfx::Rect ca_overlay_damage_rect_;
-  gfx::Rect previous_frame_full_bounding_rect_;
 
  protected:
   // Protected for testing.