Avoid calling isFocusable() during updateLayout.

third_party/WebKit/Source/core/dom/Document.cpp

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 1999 Antti Koivisto (koivisto@kde.org)
  *           (C) 2001 Dirk Mueller (mueller@kde.org)
  *           (C) 2006 Alexey Proskuryakov (ap@webkit.org)
  * Copyright (C) 2004, 2005, 2006, 2007, 2008, 2009, 2011, 2012 Apple Inc. All rights reserved.
  * Copyright (C) 2008, 2009 Torch Mobile Inc. All rights reserved. (http://www.torchmobile.com/)
  * Copyright (C) 2008, 2009, 2011, 2012 Google Inc. All rights reserved.
  * Copyright (C) 2010 Nokia Corporation and/or its subsidiary(-ies)
  * Copyright (C) Research In Motion Limited 2010-2011. All rights reserved.
@@ skipping 1713 matching lines @@
     updateStyle();
 
     notifyLayoutTreeOfSubtreeChanges();
 
     // As a result of the style recalculation, the currently hovered element might have been
     // detached (for example, by setting display:none in the :hover style), schedule another mouseMove event
     // to check if any other elements ended up under the mouse pointer due to re-layout.
     if (hoverNode() && !hoverNode()->layoutObject() && frame())
         frame()->eventHandler().dispatchFakeMouseMoveEventSoon();
 
-    if (m_focusedElement && !m_focusedElement->isFocusable())

[tkent, 2016/07/21 00:38:50]

At this line, we already updated style and layout tree, didn't we?
Why we hit the DCHECK failure in isFocusable()?

[kochi, 2016/07/21 01:32:49]

Hmm, yes.
Yesterday I saw another case that needsUpdateLayout flag is not cleared
after layout : https://bugs.chromium.org/p/chromium/issues/detail?id=605746
Maybe some other layout parts introduced a bug recently.

+    if (m_focusedElement)

[esprehn, 2016/07/23 08:37:25]

Note that we can't land this change anyway, since clearFocusedElementSoon()
posts a task. This change would have made all calls to
updateStyleAndLayoutTree() post an unconditional additional task.

not lgtm

         clearFocusedElementSoon();
     layoutViewItem().clearHitTestCache();
 
     DCHECK(!DocumentAnimations::needsAnimationTimingUpdate(*this));
 
     unsigned elementCount = styleEngine().styleForElementCount() - startElementCount;
 
     TRACE_EVENT_END1("blink,devtools.timeline", "UpdateLayoutTree", "elementCount", elementCount);
 
 #if DCHECK_IS_ON()
@@ skipping 4294 matching lines @@
 }
 
 void showLiveDocumentInstances()
 {
     WeakDocumentSet& set = liveDocumentSet();
     fprintf(stderr, "There are %u documents currently alive:\n", set.size());
     for (Document* document : set)
         fprintf(stderr, "- Document %p URL: %s\n", document, document->url().getString().utf8().data());
 }
 #endif