Improve overdraw with multiple windows

cc/base/simple_enclosed_region.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/base/simple_enclosed_region.h"
 
 #include <stddef.h>
 #include <stdint.h>
 
 #include "base/logging.h"
 #include "cc/base/region.h"
+#include "ui/gfx/geometry/rect.h"
 
 namespace cc {
 
 static bool RectIsLargerArea(const gfx::Rect& a, const gfx::Rect b) {
   int64_t a_area = static_cast<int64_t>(a.width()) * a.height();
   int64_t b_area = static_cast<int64_t>(b.width()) * b.height();
   return a_area > b_area;
 }
 
 SimpleEnclosedRegion::SimpleEnclosedRegion(const Region& region) {
@@ skipping 95 matching lines @@
     if (right > new_right && left <= new_right)
       new_right = right;
   } else if (left <= new_left && right >= new_right) {
     if (top < new_top && bottom >= new_top)
       new_top = top;
     if (bottom > new_bottom && top <= new_bottom)
       new_bottom = bottom;
   }
 
   rect_.SetRect(left, top, right - left, bottom - top);
-
+  int64_t rect_area = static_cast<int64_t>(rect_.width()) * rect_.height();
   gfx::Rect adjusted_new_rect(
       new_left, new_top, new_right - new_left, new_bottom - new_top);
-  if (RectIsLargerArea(adjusted_new_rect, rect_))
+  int64_t adjust_new_rect_area =
+      static_cast<int64_t>(adjusted_new_rect.width()) *
+      adjusted_new_rect.height();
+  gfx::Rect overlap = gfx::IntersectRects(rect_, adjusted_new_rect);
+  int64_t overlap_area =
+      static_cast<int64_t>(overlap.width()) * overlap.height();
+
+  // Based on the assumption that as we compute occlusion, each step is
+  // more likely to be occluded by things added to this region more recently due
+  // to the way we build scenes with overlapping elements adjacent to each other
+  // in the Z order. So, the area of the new rect has a weight of 2 in the
+  // weighted area calculation.
+  if (adjust_new_rect_area * 2 > rect_area + overlap_area)
     rect_ = adjusted_new_rect;
 }
 
 gfx::Rect SimpleEnclosedRegion::GetRect(size_t i) const {
   DCHECK_LT(i, GetRegionComplexity());
   return rect_;
 }
 
 }  // namespace cc