Exclude position:absolute elements when picking an anchor for ScrollAnchoring

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/frame/UseCounter.h"
 #include "core/layout/LayoutView.h"
 #include "core/layout/line/InlineTextBox.h"
@@ skipping 87 matching lines @@
     if (scroller->isFrameView())
         relativeBounds.moveBy(-flooredIntPoint(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)
+static bool candidateMayMoveWithScroller(const LayoutObject* candidate, const ScrollableArea* scroller)
 {
-    if (candidate->style() && candidate->style()->hasViewportConstrainedPosition())
+    const ComputedStyle* style = candidate->style();
+    if (style && (style->hasViewportConstrainedPosition() || style->hasOutOfFlowPosition())) {
+        // Elements with absolute positioning are often used for social nav

[skobes, 2016/05/28 00:18:10]

"social nav bars" is too specific, just say that these elements may stick to the
viewport.

[ymalik, 2016/05/31 22:47:02]

Done.

+        // bars that move with the viewort.
         return false;
+    }
 
     bool skippedByContainerLookup = false;
     candidate->container(scrollerLayoutBox(scroller), &skippedByContainerLookup);

[skobes, 2016/05/28 00:18:10]

I think we don't need this anymore?

[ymalik, 2016/05/30 18:13:03]

You're right that we don't need this if we don't special case position:absolute
with non-zero top/left. If we do end up only excluding absolute positioned
elements with non-zero top/left, then we still need this because the candidate
can be stuck to the viewport when top/left = 0.

[ymalik, 2016/05/31 22:47:01]

As per your comment below and our offline conversation, we decided to keep
top/left, so we need this.

     return !skippedByContainerLookup;
 }
 
 ScrollAnchor::ExamineResult ScrollAnchor::examine(const LayoutObject* candidate) const
 {
     if (candidate->isLayoutInline())
         return ExamineResult(Continue);
 
     if (!candidate->isText() && !candidate->isBox())
         return ExamineResult(Skip);
 
-    if (!candidateMovesWithScroller(candidate, m_scroller))
+    if (!candidateMayMoveWithScroller(candidate, m_scroller))
         return ExamineResult(Skip);
 
     LayoutRect candidateRect = relativeBounds(candidate, m_scroller);
     LayoutRect visibleRect = scrollerLayoutBoxItem(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(candidate));
@@ skipping 45 matching lines @@
         return;
 
     m_current.m_anchorObject->setIsScrollAnchorObject();
     m_current.m_savedRelativeOffset = computeRelativeOffset(
         m_current.m_anchorObject, m_scroller, m_current.m_corner);
 
     if (m_lastAdjusted) {
         // We need to update m_lastAdjusted.m_savedRelativeOffset, since it is
         // relative to the visible rect and the user may have scrolled since the
         // last adjustment.
-        if (!candidateMovesWithScroller(m_lastAdjusted.m_anchorObject, m_scroller)) {
+        if (!candidateMayMoveWithScroller(m_lastAdjusted.m_anchorObject, m_scroller)) {
             m_lastAdjusted.clear();
         } else if (m_lastAdjusted.m_anchorObject == m_current.m_anchorObject
             && m_lastAdjusted.m_corner == m_current.m_corner) {
             m_lastAdjusted.m_savedRelativeOffset = m_current.m_savedRelativeOffset;
         } else {
             m_lastAdjusted.m_savedRelativeOffset = computeRelativeOffset(
                 m_lastAdjusted.m_anchorObject, m_scroller, m_lastAdjusted.m_corner);
         }
     }
 }
@@ skipping 91 matching lines @@
 
 void ScrollAnchor::notifyRemoved(LayoutObject* layoutObject)
 {
     if (m_current.m_anchorObject == layoutObject)
         m_current.clear();
     if (m_lastAdjusted.m_anchorObject == layoutObject)
         m_lastAdjusted.clear();
 }
 
 } // namespace blink