Percent-width blocks cannot form a re-layout boundary.

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/)
@@ skipping 698 matching lines @@
   // LayoutInline can't be relayout roots since LayoutBlockFlow is responsible
   // for layouting them.
   if (object->isLayoutInline())
     return false;
 
   // Table parts can't be relayout roots since the table is responsible for
   // layouting all the parts.
   if (object->isTablePart())
     return false;
 
-  if (object->style()->containsLayout() && object->style()->containsSize())
+  const ComputedStyle* style = object->style();
+  if (style->containsLayout() && style->containsSize())
     return true;
 
   if (!object->hasOverflowClip())
     return false;
 
-  if (object->style()->width().isIntrinsicOrAuto() ||
-      object->style()->height().isIntrinsicOrAuto() ||
-      object->style()->height().isPercentOrCalc())
+  // If either dimension is percent-based, intrinsic, or anything but fixed,
+  // this object cannot form a re-layout boundary. A non-fixed computed logical
+  // height will allow the object to grow and shrink based on the content
+  // inside. The same goes for for logical width, if this objects is inside a
+  // shrink-to-fit container, for instance.
+  if (!style->width().isFixed() || !style->height().isFixed())
     return false;
 
   // Scrollbar parts can be removed during layout. Avoid the complexity of
   // having to deal with that.
   if (object->isLayoutScrollbarPart())
     return false;
 
   // In general we can't relayout a flex item independently of its container;
   // not only is the result incorrect due to the override size that's set, it
   // also messes with the cached main size on the flexbox.
@@ skipping 2770 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