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

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

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 2004-2005 Allan Sandfeld Jensen (kde@carewolf.com)
  * Copyright (C) 2006, 2007 Nicholas Shanks (webkit@nickshanks.com)
  * Copyright (C) 2005, 2006, 2007, 2008, 2009, 2010, 2011, 2012, 2013 Apple Inc.
  * All rights reserved.
  * Copyright (C) 2007 Alexey Proskuryakov <ap@webkit.org>
  * Copyright (C) 2007, 2008 Eric Seidel <eric@webkit.org>
  * Copyright (C) 2008, 2009 Torch Mobile Inc. All rights reserved.
  * (http://www.torchmobile.com/)
@@ skipping 494 matching lines @@
     // SVG text layout code expects us to be a block-level style element.
     if ((isSVGForeignObjectElement(*element) || isSVGTextElement(*element)) &&
         style.IsDisplayInlineType())
       style.SetDisplay(EDisplay::kBlock);
 
     // Columns don't apply to svg text elements.
     if (isSVGTextElement(*element))
       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