Exclude position:absolute elements when picking an anchor for ScrollAnchoring

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 54a7f24099bfe08a2ab56c432cb2020c2f8fdb9c..c06566f16382efe14278af18b90c944653446faa 100644
--- a/third_party/WebKit/Source/core/layout/ScrollAnchor.cpp
+++ b/third_party/WebKit/Source/core/layout/ScrollAnchor.cpp
@@ -105,10 +105,14 @@ static LayoutPoint computeRelativeOffset(const LayoutObject* layoutObject, const
     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.

@@ -123,7 +127,7 @@ ScrollAnchor::ExamineResult ScrollAnchor::examine(const LayoutObject* candidate)
     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);
@@ -189,7 +193,7 @@ void ScrollAnchor::save()
         // 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) {