Handle nested position:sticky elements correctly (compositor)

cc/layers/layer_sticky_position_constraint.h

 // Copyright 2013 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.
 
 #ifndef CC_LAYERS_LAYER_STICKY_POSITION_CONSTRAINT_H_
 #define CC_LAYERS_LAYER_STICKY_POSITION_CONSTRAINT_H_
 
 #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 {
 
 struct CC_EXPORT LayerStickyPositionConstraint {
   LayerStickyPositionConstraint();
   LayerStickyPositionConstraint(const LayerStickyPositionConstraint& other);
 
   bool is_sticky : 1;
   bool is_anchored_left : 1;
   bool is_anchored_right : 1;
   bool is_anchored_top : 1;
   bool is_anchored_bottom : 1;
 
   // The offset from each edge of the ancestor scroller (or the viewport) to
   // try to maintain to the sticky box as we scroll.
   float left_offset;
   float right_offset;
   float top_offset;
   float bottom_offset;
 
   // 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;
 };
 
 }  // namespace cc
 
 #endif  // CC_LAYERS_LAYER_STICKY_POSITION_CONSTRAINT_H_