Make abspos <dialog>'s containing block be the ICB.

Source/core/html/HTMLDialogElement.cpp

 /*
  * Copyright (C) 2012 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  *
  * 1.  Redistributions of source code must retain the above copyright
  *     notice, this list of conditions and the following disclaimer.
  * 2.  Redistributions in binary form must reproduce the above copyright
@@ skipping 74 matching lines @@
     document()->updateLayoutIgnorePendingStylesheets();
 
     RenderBox* box = renderBox();
     if (!box || !needsCenteredPositioning(box->style()))
         return;
 
     // Set up dialog's position to be safe-centered in the viewport.
     // FIXME: Figure out what to do in vertical writing mode.
     FrameView* frameView = document()->view();
     int scrollTop = frameView->scrollOffset().height();
-    FloatPoint absolutePoint(0, scrollTop);
     int visibleHeight = frameView->visibleContentRect(ScrollableArea::IncludeScrollbars).height();
+    m_top = scrollTop;
     if (box->height() < visibleHeight)
-        absolutePoint.move(0, (visibleHeight - box->height()) / 2);
-    FloatPoint localPoint = box->containingBlock()->absoluteToLocal(absolutePoint);
-
-    m_top = localPoint.y();
+        m_top += (visibleHeight - box->height()) / 2;
     m_topIsValid = true;
 
     // FIXME: It's inefficient to reattach here. We could do better by mutating style directly and forcing another layout.
     reattach();
 }
 
 void HTMLDialogElement::show()
 {
     if (fastHasAttribute(openAttr))
         return;
@@ skipping 15 matching lines @@
 bool HTMLDialogElement::isPresentationAttribute(const QualifiedName& name) const
 {
     // FIXME: Workaround for <https://bugs.webkit.org/show_bug.cgi?id=91058>: modifying an attribute for which there is an attribute selector
     // in html.css sometimes does not trigger a style recalc.
     if (name == openAttr)
         return true;
 
     return HTMLElement::isPresentationAttribute(name);
 }
 
+bool HTMLDialogElement::shouldBeReparentedUnderRenderView(const RenderStyle* style) const
+{
+    if (style && style->position() == AbsolutePosition)

[Julien - ping for review, 2013/05/10 21:04:56]

This should probably be needsCenteredPositioning to match the rest of the code.

[falken, 2013/05/13 03:04:56]

needsCenteredPositioning only returns true for auto 'top' and 'bottom'. I think
we want the containing block of all abspos dialogs to be the ICB, even if
top/bottom are not auto.

+        return true;
+    return Element::shouldBeReparentedUnderRenderView(style);
+}
+
 } // namespace WebCore