Handle position:absolute candidates correctly in ScrollAnchor.

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

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "core/layout/ScrollAnchor.h"
 
 #include "core/frame/FrameView.h"
 #include "core/layout/LayoutView.h"
 #include "core/layout/line/InlineTextBox.h"
 #include "core/paint/PaintLayerScrollableArea.h"
@@ skipping 68 matching lines @@
     if (scroller->isFrameView())
         relativeBounds.moveBy(-LayoutPoint(scroller->scrollPositionDouble()));
     return relativeBounds;
 }
 
 static LayoutPoint computeRelativeOffset(const LayoutObject* layoutObject, const ScrollableArea* scroller, Corner corner)
 {
     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())
         return ExamineResult(Continue);
 
     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);
     LayoutRect visibleRect = scrollerLayoutBox(m_scroller)->overflowClipRect(LayoutPoint());
 
     bool occupiesSpace = candidateRect.width() > 0 && candidateRect.height() > 0;
     if (occupiesSpace && visibleRect.intersects(candidateRect)) {
         return ExamineResult(
             visibleRect.contains(candidateRect) ? Return : Constrain,
             cornerFromCandidateRect(candidateRect, visibleRect));
@@ skipping 61 matching lines @@
 void ScrollAnchor::clear()
 {
     LayoutObject* anchorObject = m_anchorObject;
     m_anchorObject = nullptr;
 
     if (anchorObject)
         anchorObject->maybeClearIsScrollAnchorObject();
 }
 
 } // namespace blink