[Ozone] Fix SingleOnTop issue by checking if the quad is clipped.

cc/output/overlay_strategy_single_on_top.cc

Index: cc/output/overlay_strategy_single_on_top.cc
diff --git a/cc/output/overlay_strategy_single_on_top.cc b/cc/output/overlay_strategy_single_on_top.cc
index fd47e0f3b721096f55b0a531f47e75233b3926ff..a45787a2459e3e3279c2b7fac3181a53c9f7359d 100644
--- a/cc/output/overlay_strategy_single_on_top.cc
+++ b/cc/output/overlay_strategy_single_on_top.cc
@@ -29,6 +29,11 @@ bool OverlayStrategySingleOnTop::TryOverlay(
   gfx::RectF rect = draw_quad->rect;
   draw_quad->shared_quad_state->quad_to_target_transform.TransformRect(&rect);
 
+  // Check if the quad is clipped.
+  if (draw_quad->shared_quad_state->visible_quad_layer_rect.size() !=

[alexst (slow to review), 2015/08/27 12:42:36]

I don't think this is the right place to check for it. For example, this would
work fine on MacOS.

OverlayCandidate should already contain enough info in it to figure out this
scenario (if not, we should add what's missing), and instead of rejecting it
here,we need to reject it as part of line 58  
capability_checker->CheckOverlaySupport(&candidates);

Which on ChromeOS will go to

https://code.google.com/p/chromium/codesearch#chromium/src/ui/ozone/platform/...

and some place else on other platforms.

+      draw_quad->shared_quad_state->quad_layer_bounds)

[danakj, 2015/08/27 17:37:54]

Can you just use the draw_quad->shared_quad_state->is_clipped bool?

[william.xie1, 2015/08/28 00:00:46]

Hi Dana, I tried it, but is_clipped was always true, it maybe used for other
purpose?

[danakj, 2015/08/28 22:55:05]

Ah, ok well even if it's always true, it won't always clip something.

To determine if a quad is clipped the correct method would be to map the quad's
|rect| with the quad_to_target_transform and verify if the clip_rect.Contains()
that mapped rect. If not, then it will get clipped.

And if is_clipped is false you don't need to do any of that.

+    return false;
+
   // Check that no prior quads overlap it.
   for (auto overlap_iter = quad_list.cbegin();
        overlap_iter != candidate_iterator; ++overlap_iter) {