Improve overdraw with multiple windows

cc/trees/occlusion_tracker_unittest.cc

Index: cc/trees/occlusion_tracker_unittest.cc
diff --git a/cc/trees/occlusion_tracker_unittest.cc b/cc/trees/occlusion_tracker_unittest.cc
index 8389954958e4689b8ff28fc37e6b81cb151a878e..d80365e46dfb66031834b2b106ba33220f5a2a7c 100644
--- a/cc/trees/occlusion_tracker_unittest.cc
+++ b/cc/trees/occlusion_tracker_unittest.cc
@@ -1463,10 +1463,10 @@ class OcclusionTrackerTestPixelsNeededForDropShadowBackgroundFilter
       gfx::Rect expected_occlusion;
       switch (i) {
         case LEFT:
-          // The right half of the occlusion is close enough to cast a shadow

[danakj, 2017/05/15 21:47:43]

I think this part of the comment should be kept, explaining that the shadow is
visible in the background filter.

[yiyix, 2017/05/17 03:00:32]

Done.

-          // that would be visible in the background filter. The shadow reaches
-          // 3*5 + 10 = 25 pixels to the right.

[danakj, 2017/05/15 21:47:43]

But I don't actually understand what's shrinking the rect more from what you
wrote there. Theres a (0,0,50,200) area that could occlude. The shadow shrinks
that on the left side by 25. But what shrinks it from the right and bottom
sides?

Did we pick something else to use as the occlusion instead of |occluding_layer|?

[yiyix, 2017/05/17 03:00:32]

From what I understand, there are 2 rects: occluding_layer: (0, 0, 50. 200) &&
filtered_surface: (50, 50, 100, 100). The filter surface is first transformed to
(50, 50, 50, 50) by |scale_by_half|, then it become (25, 25, 80, 80) after
applying filter. After "expanding the non-opaque pixels outside the rect", the
rect in filter surface become (25, 25, 20, 80). This process is not explained in
the original test description. I can put some transformation details in the
description.

-          expected_occlusion = gfx::Rect(0, 0, 25, 200);
+          // The occlusion needs for the shadow reaches 3*5+10 pixel to the

[danakj, 2017/05/15 21:47:42]

pixels

[yiyix, 2017/05/17 03:00:32]

Done.

+          // right and the rect in filtered_surface which is (25, 25, 20*80), so

[danakj, 2017/05/15 21:47:43]

20, 80

[yiyix, 2017/05/17 03:00:32]

Done.

+          // the expected occlusion is (0, 25, 45*80).

[danakj, 2017/05/15 21:47:43]

45, 80  (it reads like a triple with a product otherwise without more context)

[yiyix, 2017/05/17 03:00:32]

Done.

+          expected_occlusion = gfx::Rect(0, 25, 45, 80);
           break;
         case RIGHT:
           // The shadow spreads 3*5 - 10 = 5 pixels to the left, so the
@@ -1475,7 +1475,7 @@ class OcclusionTrackerTestPixelsNeededForDropShadowBackgroundFilter
           break;
         case TOP:
           // Similar to LEFT.
-          expected_occlusion = gfx::Rect(0, 0, 200, 25);
+          expected_occlusion = gfx::Rect(25, 0, 80, 45);
           break;
         case BOTTOM:
           // Similar to RIGHT.