Disable anchoring if ancestor changes padding or margin

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 d16bbed3575f12d768200941106ed21011b8e089..3e9f636fcac1a2ea744e1a4f8fd3edd0501fcec1 100644
--- a/third_party/WebKit/Source/core/layout/ScrollAnchor.cpp
+++ b/third_party/WebKit/Source/core/layout/ScrollAnchor.cpp
@@ -19,6 +19,7 @@ static const int kMaxAdjustments = 20;
 ScrollAnchor::ScrollAnchor()
     : m_hasBounced(false)
     , m_adjustmentCount(0)
+    , m_anyAncestorChangedPaddingOrMargin(false)
 {
 }
 
@@ -199,13 +200,18 @@ void ScrollAnchor::save()
         clear();
         return;
     }
-    if (m_current)
+
+    if (m_current) {
+        m_anyAncestorChangedPaddingOrMargin = paddingOrBorderChanged();
         return;
+    }
 
     findAnchor();
     if (!m_current)
         return;
 
+    m_anyAncestorChangedPaddingOrMargin = paddingOrBorderChanged();
+
     m_current.m_anchorObject->setIsScrollAnchorObject();
     m_current.m_savedRelativeOffset = computeRelativeOffset(
         m_current.m_anchorObject, m_scroller, m_current.m_corner);
@@ -240,6 +246,18 @@ IntSize ScrollAnchor::computeAdjustment(const AnchorPoint& anchorPoint) const
         roundedIntSize(anchorPoint.m_savedRelativeOffset);
 }
 
+bool ScrollAnchor::paddingOrBorderChanged() const
+{
+    LayoutObject* current = anchorObject();
+    while (current && current != scrollerLayoutBox(m_scroller)) {
+        if (current->isLayoutBlock()
+            && (toLayoutBlock(current)->heightAvailableToChildrenChanged()))
+            return true;
+        current = current->parent();
+    }
+    return false;
+}
+
 void ScrollAnchor::restore()
 {
     DCHECK(m_scroller);
@@ -250,11 +268,23 @@ void ScrollAnchor::restore()
         adjust(-m_lastAdjustment);
         return;
     }
+
     if (!m_current)
         return;
+
+    // Authors often change the padding or margin when trying to simulate
+    // position:sticky on headers. Use this as a heuristic to disable scroll
+    // anchoring so that we don't conflict with JS changes.
+    // TODO(ymalik): Does this change allow us to remove the de-bouncing hack?
+    if (m_anyAncestorChangedPaddingOrMargin) {
+        clear();
+        return;
+    }
+
     IntSize adjustment = computeAdjustment(m_current);
     if (adjustment.isZero())
         return;
+
     if (adjustment == -m_lastAdjustment && m_hasBounced) {
         // Don't bounce more than once.
         clear();