Handle nested position:sticky elements correctly (compositor)

cc/layers/layer_sticky_position_constraint.h

Index: cc/layers/layer_sticky_position_constraint.h
diff --git a/cc/layers/layer_sticky_position_constraint.h b/cc/layers/layer_sticky_position_constraint.h
index 3f43bc7751ab614b2c12b25c18d73a71cd308012..beb841d908d659ae157dea2b7490458e55304b13 100644
--- a/cc/layers/layer_sticky_position_constraint.h
+++ b/cc/layers/layer_sticky_position_constraint.h
@@ -7,7 +7,9 @@
 
 #include "cc/base/cc_export.h"
 
-#include "ui/gfx/geometry/rect.h"
+#include "ui/gfx/geometry/point_f.h"
+#include "ui/gfx/geometry/rect_f.h"
+#include "ui/gfx/geometry/vector2d_f.h"
 
 namespace cc {
 
@@ -31,18 +33,37 @@ struct CC_EXPORT LayerStickyPositionConstraint {
   // The layout offset of the sticky box relative to its containing layer.
   // This is used to detect the sticky offset the main thread has applied
   // to the layer.
-  gfx::Point parent_relative_sticky_box_offset;
+  gfx::PointF parent_relative_sticky_box_offset;
 
   // The rectangle corresponding to original layout position of the sticky box
   // relative to the scroll ancestor. The sticky box is only offset once the
   // scroll has passed its initial position (e.g. top_offset will only push
   // the element down from its original position).
-  gfx::Rect scroll_container_relative_sticky_box_rect;
+  gfx::RectF scroll_container_relative_sticky_box_rect;
 
   // The layout rectangle of the sticky box's containing block relative to the
   // scroll ancestor. The sticky box is only moved as far as its containing
   // block boundary.
-  gfx::Rect scroll_container_relative_containing_block_rect;
+  gfx::RectF scroll_container_relative_containing_block_rect;
+
+  // The nearest ancestor sticky layers that affect the sticky box constraint

[flackr, 2017/03/17 15:01:55]

Mention layer id in the comment, and -1 for no such layer.

[smcgruer, 2017/03/17 17:36:51]

Done.

+  // rect and the containing block constraint rect respectively.
+  int nearest_layer_shifting_sticky_box;
+  int nearest_layer_shifting_containing_block;
+
+  // For performance we cache our accumulated sticky offset to allow descendant
+  // sticky elements to offset their constraint rects. Because we can either
+  // affect the sticky box constraint rect or the containing block constraint
+  // rect, we need to accumulate both.
+  gfx::Vector2dF total_sticky_box_sticky_offset;
+  gfx::Vector2dF total_containing_block_sticky_offset;
+
+  // Returns the nearest sticky ancestor layer, or null if no such layer exists.

[flackr, 2017/03/17 15:01:55]

s/null/-1 ?

[smcgruer, 2017/03/17 17:36:51]

Done.

+  int NearestStickyAncestor() {
+    return (nearest_layer_shifting_sticky_box >= 0)
+               ? nearest_layer_shifting_sticky_box
+               : nearest_layer_shifting_containing_block;
+  }
 
   bool operator==(const LayerStickyPositionConstraint&) const;
   bool operator!=(const LayerStickyPositionConstraint&) const;