Handle nested position:sticky elements correctly (compositor)

cc/trees/draw_property_utils.cc

Index: cc/trees/draw_property_utils.cc
diff --git a/cc/trees/draw_property_utils.cc b/cc/trees/draw_property_utils.cc
index a591845f38b5c02e7e5bde6a6c134e076acbc110..794b8c736fe6cd6623715633a10468b092166615 100644
--- a/cc/trees/draw_property_utils.cc
+++ b/cc/trees/draw_property_utils.cc
@@ -1475,6 +1475,41 @@ void UpdateElasticOverscroll(PropertyTrees* property_trees,
                                   elastic_overscroll);
 }
 
+gfx::Vector2dF CalculateTotalStickyOffsetToScroller(
+    Layer* layer,
+    int scroll_ancestor_layer_id) {
+  // Find nearest ancestor which is sticky, up to the scroller.
+  while (layer && !layer->sticky_position_constraint().is_sticky) {
+    layer = layer->parent();
+    if (!layer || layer->id() == scroll_ancestor_layer_id)
+      return gfx::Vector2dF();
+  }
+
+  // Walk all sticky layers from the current layer to the ancestor scroller
+  // layer. We achieve this by walking the |nearest_layer_shifting_sticky_box|
+  // when available (which is the nearest sticky element between us and our
+  // containing block), and |nearest_layer_shifting_containing_block| otherwise
+  // (which is the nearest sticky element between our containing block and the
+  // ancestor scroller).
+  //
+  // Once both of these are null, we have no more sticky ancestors to our scroll
+  // ancestor layer.
+  LayerStickyPositionConstraint constraints =
+      layer->sticky_position_constraint();
+  gfx::Vector2dF sticky_to_remove(constraints.local_sticky_offset);
+  Layer* nextSticky = constraints.nearest_layer_shifting_sticky_box
+                          ? constraints.nearest_layer_shifting_sticky_box
+                          : constraints.nearest_layer_shifting_containing_block;
+  while (nextSticky) {
+    constraints = nextSticky->sticky_position_constraint();
+    sticky_to_remove += constraints.local_sticky_offset;
+    nextSticky = constraints.nearest_layer_shifting_sticky_box
+                     ? constraints.nearest_layer_shifting_sticky_box
+                     : constraints.nearest_layer_shifting_containing_block;
+  }
+  return sticky_to_remove;
+}
+
 }  // namespace draw_property_utils
 
 }  // namespace cc