Handle position:absolute candidates correctly in ScrollAnchor.

third_party/WebKit/Source/core/layout/ScrollAnchor.cpp

Index: third_party/WebKit/Source/core/layout/ScrollAnchor.cpp
diff --git a/third_party/WebKit/Source/core/layout/ScrollAnchor.cpp b/third_party/WebKit/Source/core/layout/ScrollAnchor.cpp
index ba2558be4ea525a692f070152ea8065f091c6b32..7959b41f6a48b238cf4a8b6d07280beb69333687 100644
--- a/third_party/WebKit/Source/core/layout/ScrollAnchor.cpp
+++ b/third_party/WebKit/Source/core/layout/ScrollAnchor.cpp
@@ -86,6 +86,16 @@ static LayoutPoint computeRelativeOffset(const LayoutObject* layoutObject, const
     return cornerPointOfRect(relativeBounds(layoutObject, scroller), corner);
 }
 
+static bool candidateMovesWithScroller(const LayoutObject* candidate, const ScrollableArea* scroller)
+{
+    if (candidate->style() && candidate->style()->hasViewportConstrainedPosition())
+        return false;
+
+    bool skippedByContainerLookup = false;
+    candidate->container(scrollerLayoutBox(scroller), &skippedByContainerLookup);
+    return !skippedByContainerLookup;

[ymalik, 2016/03/25 18:53:19]

Sorry, could you briefly explain why if !skippedByContainerLookup then we know
that candidate moves with scroller?

[skobes, 2016/03/25 19:36:35]

This is looking for cases where the candidate's "containing block" as defined by
CSS is outside of the scroller.

For example, the containing block of a position:absolute element is the nearest
"positioned" (i.e. not static) ancestor.  LayoutObject::container will continue
walking up the DOM until it finds an ancestor that
canContainAbsolutePositionObjects().

If that ancestor is outside of the scroller, the candidate's position will not
be affected by the scroll offset.

See: https://jsbin.com/lomocu/edit?html,output

+}
+
 ScrollAnchor::ExamineResult ScrollAnchor::examine(const LayoutObject* candidate) const
 {
     if (candidate->isLayoutInline())
@@ -94,7 +104,7 @@ ScrollAnchor::ExamineResult ScrollAnchor::examine(const LayoutObject* candidate)
     if (!candidate->isText() && !candidate->isBox())
         return ExamineResult(Skip);
 
-    if (candidate->style() && candidate->style()->hasViewportConstrainedPosition())
+    if (!candidateMovesWithScroller(candidate, m_scroller))
         return ExamineResult(Skip);
 
     LayoutRect candidateRect = relativeBounds(candidate, m_scroller);