Disable scroll anchoring is an element within the scroll changes its in-flow state

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

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 1999 Antti Koivisto (koivisto@kde.org)
  *           (C) 2000 Dirk Mueller (mueller@kde.org)
  *           (C) 2004 Allan Sandfeld Jensen (kde@carewolf.com)
  * Copyright (C) 2004, 2005, 2006, 2007, 2008, 2011 Apple Inc. All rights reserved.
  * Copyright (C) 2009 Google Inc. All rights reserved.
  * Copyright (C) 2009 Torch Mobile Inc. All rights reserved. (http://www.torchmobile.com/)
  *
  * This library is free software; you can redistribute it and/or
@@ skipping 1694 matching lines @@
 {
     ASSERT(a->cursors() != b->cursors());
     return a->cursors() && b->cursors() && *a->cursors() == *b->cursors();
 }
 
 static inline bool areCursorsEqual(const ComputedStyle* a, const ComputedStyle* b)
 {
     return a->cursor() == b->cursor() && (a->cursors() == b->cursors() || areNonIdenticalCursorListsEqual(a, b));
 }
 
+void LayoutObject::setScrollAnchorDisablingStyleChangedOnAncestor()
+{
+    // Walk up the parent chain and find the first scrolling block to disable
+    // scroll anchoring on.
+    LayoutObject* object = parent();
+    Element* viewportDefiningElement = document().viewportDefiningElement();
+    while (object) {
+        if (object->isLayoutBlock()) {
+            LayoutBlock* block = toLayoutBlock(object);
+            if (block->hasOverflowClip() || block->node() == viewportDefiningElement) {
+                block->setScrollAnchorDisablingStyleChanged(true);
+                return;
+            }
+        }
+        object = object->parent();
+    }
+}
+
 void LayoutObject::styleDidChange(StyleDifference diff, const ComputedStyle* oldStyle)
 {
     if (s_affectsParentBlock)
         handleDynamicFloatPositionChange(this);
 
     if (!m_parent)
         return;
 
     if (diff.needsFullLayout()) {
         LayoutCounter::layoutObjectStyleChanged(*this, oldStyle, *m_style);
 
+        // If the in-flow state of an element is changed, disable scroll
+        // anchoring on the containing scroller.
+        if (oldStyle->hasOutOfFlowPosition() != m_style->hasOutOfFlowPosition())
+            setScrollAnchorDisablingStyleChangedOnAncestor();
+
         // If the object already needs layout, then setNeedsLayout won't do
         // any work. But if the containing block has changed, then we may need
         // to mark the new containing blocks for layout. The change that can
         // directly affect the containing block of this object is a change to
         // the position style.
         if (needsLayout() && oldStyle->position() != m_style->position())
             markContainerChainForLayout();
 
         // Ditto.
         if (needsOverflowRecalcAfterStyleChange() && oldStyle->position() != m_style->position())
@@ skipping 1497 matching lines @@
         const blink::LayoutObject* root = object1;
         while (root->parent())
             root = root->parent();
         root->showLayoutTreeAndMark(object1, "*", object2, "-", 0);
     } else {
         WTFLogAlways("%s", "Cannot showLayoutTree. Root is (nil)");
     }
 }
 
 #endif