Clean compositing inputs for location APIs for sticky-affected elements.

third_party/WebKit/Source/core/css/resolver/StyleAdjuster.cpp

Index: third_party/WebKit/Source/core/css/resolver/StyleAdjuster.cpp
diff --git a/third_party/WebKit/Source/core/css/resolver/StyleAdjuster.cpp b/third_party/WebKit/Source/core/css/resolver/StyleAdjuster.cpp
index 57ce4f32a3654a19edf93e9414c22d1235eb31e9..d3969a25311bab1df679bcd78baa591ea54c1ad0 100644
--- a/third_party/WebKit/Source/core/css/resolver/StyleAdjuster.cpp
+++ b/third_party/WebKit/Source/core/css/resolver/StyleAdjuster.cpp
@@ -512,6 +512,13 @@ void StyleAdjuster::AdjustComputedStyle(
       style.ClearMultiCol();
   }
   AdjustStyleForAlignment(style, layout_parent_style);
+
+  // Once the position of the element has been decided, we need to track whether
+  // or not it is in a sticky sub-tree. This is necessary to properly handle the
+  // document lifecycle for certain script APIs, see
+  // Document::EnsureLifecycleValidForLocationAPIsForNode.
+  style.SetIsInStickySubtree(parent_style.IsInStickySubtree() ||

[fs, 2017/05/01 21:19:34]

if (style.GetPosition() == EPosition::kSticky)
  style.SetIsInStickySubtree(true);

should be enough here (inheritance has happened already, so the parent-check is
redundant.)

[smcgruer, 2017/05/02 15:25:41]

I'm not sure I follow here. If our parent/ancestor is sticky, we may not be
sticky but still want to be marked as in a sticky sub-tree, e.g.:

<div id="parent" style="position: sticky">
  <div id="child"></div>
</div>

Here, we want *both* parent and child to have IsInStickySubtree() return true,
but only parent will have style.GetPosition() == EPosition::kSticky . The child
would have style.GetPosition() == EPosition::kStatic afaik.

[fs, 2017/05/02 15:58:07]

If parent is sticky, the inheritance mechanism will have marked the child as
being in a sticky subtree already before we even get here. So here we only care
about the condition that (potentially, I take it 'sticky' within 'sticky' is
rare) makes the current element create a new sticky subtree - which is the '...
== EPosition::kSticky' condition. I.e that covers the false => true transition,
and all other cases are handled via inheritance.

[smcgruer, 2017/05/02 17:25:30]

Ok, the inheritance code is definitely confirmed magic at this point ;). Thanks,
removed the unnecessary part.

+                             style.GetPosition() == EPosition::kSticky);
 }
 
 }  // namespace blink