Main-thread scroll if we have fixed-pos children and overflow:hidden.

Source/core/frame/FrameView.cpp

 /*
  * Copyright (C) 1998, 1999 Torben Weis <weis@kde.org>
  *                     1999 Lars Knoll <knoll@kde.org>
  *                     1999 Antti Koivisto <koivisto@kde.org>
  *                     2000 Dirk Mueller <mueller@kde.org>
  * Copyright (C) 2004, 2005, 2006, 2007, 2008 Apple Inc. All rights reserved.
  *           (C) 2006 Graham Dennis (graham.dennis@gmail.com)
  *           (C) 2006 Alexey Proskuryakov (ap@nypop.com)
  * Copyright (C) 2009 Google Inc. All rights reserved.
  *
@@ skipping 2248 matching lines @@
 
 IntRect FrameView::scrollableAreaBoundingBox() const
 {
     RenderPart* ownerRenderer = frame().ownerRenderer();
     if (!ownerRenderer)
         return frameRect();
 
     return ownerRenderer->absoluteContentQuad().enclosingBoundingBox();
 }
 
+
 bool FrameView::isScrollable()
 {
+    return scrollingReasons() == Scrollable;
+}
+
+FrameView::ScrollingReasons FrameView::scrollingReasons()
+{
     // Check for:
     // 1) If there an actual overflow.
     // 2) display:none or visibility:hidden set to self or inherited.
     // 3) overflow{-x,-y}: hidden;
     // 4) scrolling: no;
 
     // Covers #1
     IntSize contentsSize = this->contentsSize();
     IntSize visibleContentSize = visibleContentRect().size();
     if ((contentsSize.height() <= visibleContentSize.height() && contentsSize.width() <= visibleContentSize.width()))
-        return false;
+        return NotScrollableNoOverflow;
 
     // Covers #2.
     // FIXME: Do we need to fix this for OOPI?
     HTMLFrameOwnerElement* owner = m_frame->deprecatedLocalOwner();
     if (owner && (!owner->renderer() || !owner->renderer()->visibleToHitTesting()))
-        return false;
+        return NotScrollableNotVisible;
 
     // Cover #3 and #4.
     ScrollbarMode horizontalMode;
     ScrollbarMode verticalMode;
     calculateScrollbarModesForLayoutAndSetViewportRenderer(horizontalMode, verticalMode, RulesFromWebContentOnly);
     if (horizontalMode == ScrollbarAlwaysOff && verticalMode == ScrollbarAlwaysOff)
-        return false;
+        return NotScrollableExplicitlyDisabled;
 
-    return true;
+    return Scrollable;
 }
 
 void FrameView::updateScrollableAreaSet()
 {
     // That ensures that only inner frames are cached.
     FrameView* parentFrameView = this->parentFrameView();
     if (!parentFrameView)
         return;
 
     if (!isScrollable()) {
@@ skipping 1622 matching lines @@
         return;
 
     ScrollableArea::setScrollOrigin(origin);
 
     // Update if the scroll origin changes, since our position will be different if the content size did not change.
     if (updatePositionAtAll && updatePositionSynchronously)
         updateScrollbars(scrollOffsetDouble());
 }
 
 } // namespace blink