Fix incorrect use of coords conversion for sticky elements

third_party/WebKit/Source/core/layout/compositing/CompositedLayerMapping.cpp

Index: third_party/WebKit/Source/core/layout/compositing/CompositedLayerMapping.cpp
diff --git a/third_party/WebKit/Source/core/layout/compositing/CompositedLayerMapping.cpp b/third_party/WebKit/Source/core/layout/compositing/CompositedLayerMapping.cpp
index b938ccffbe207cb53c29ea507fcb973da2d904a8..17691563d39a802e21ae22ccbdeae4e6f042108f 100644
--- a/third_party/WebKit/Source/core/layout/compositing/CompositedLayerMapping.cpp
+++ b/third_party/WebKit/Source/core/layout/compositing/CompositedLayerMapping.cpp
@@ -315,8 +315,12 @@ void CompositedLayerMapping::UpdateStickyConstraints(
     // The enclosing layers offset returned from |convertToLayerCoords| must be
     // adjusted for both scroll and ancestor sticky elements.
     LayoutPoint enclosing_layer_offset;
-    compositing_container->ConvertToLayerCoords(ancestor_overflow_layer,
-                                                enclosing_layer_offset);
+    bool is_descendant = false;

[flackr, 2017/05/17 19:44:03]

Is this not equivalent to !ScrollParent()?

[yigu, 2017/05/17 20:28:15]

No. is_descendant will be false if compositing_container is not a descendant of
ancestor_overflow_layer while !ScrollParent may be true in this case if we
composite the ancestor using will-change:transform. That way the sticky element
doesn't have a ScrollParent.

[flackr, 2017/05/18 15:54:08]

Composite which ancestor? If you apply will-change: transform to the scroller,
then the compositing container is the scroller so it will get no offset from
ConvertToLayerCoords called on itself. If will-change: transform is above the
scroller then ScrollParent will be true.

+    compositing_container->ConvertToLayerCoords(
+        ancestor_overflow_layer, enclosing_layer_offset, &is_descendant);
+    if (!is_descendant)
+      enclosing_layer_offset = LayoutPoint();

[flackr, 2017/05/17 19:44:03]

How does the sticky element know where it is with respect to the scroller if we
don't know where the scroller is with respect to the compositing container?

[yigu, 2017/05/17 20:28:15]

When the compositing_container is a descendant of the ancestor_overflow_layer as
it should be based on the logic of ConvertToLayerCoords, everything is the same
as before. But if that's not the case, the enclosing_layer_offset computed from
ConvertToLayerCoords doesn't make sense anymore therefore we should reset the
value to avoid misuse below.

[flackr, 2017/05/18 15:54:08]

My concern is when the scroller is not at the top-left corner of the compositing
container. Don't we need to add in the position of the scroller in order to find
out where the sticky_box_rect maps to in the ancestor composited layer?

e.g.
scrollContainerRelativeStickyBoxRect = (100, 100) but the scroll container is
not composited, and is at (50, 50) within compositing_container.

This means that the parent_relative_sticky_box_rect should be (150, 150) which
would seem to require adding that (50, 50) offset.

+
     DCHECK(!ScrollParent() || ScrollParent() == ancestor_overflow_layer);
     if (!ScrollParent() && compositing_container != ancestor_overflow_layer) {
       enclosing_layer_offset += LayoutSize(