There should never be unplaced floats from other blocks.

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

 /*
  * Copyright (C) 2013 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:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 3614 matching lines @@
   // The float cannot start above the top position of the last positioned float.
   if (lastPlacedFloatingObject)
     logicalTop =
         std::max(logicalTopForFloat(*lastPlacedFloatingObject), logicalTop);
 
   FloatingObjectSetIterator end = floatingObjectSet.end();
   // Now walk through the set of unpositioned floats and place them.
   for (; it != end; ++it) {
     FloatingObject& floatingObject = *it->get();
     // The containing block is responsible for positioning floats, so if we have
-    // floats in our list that come from somewhere else, do not attempt to
-    // position them.
-    if (floatingObject.layoutObject()->containingBlock() != this)
-      continue;
+    // unplaced floats in our list that come from somewhere else, we have a bug.
+    DCHECK_EQ(floatingObject.layoutObject()->containingBlock(), this);
 
     LayoutBox* childBox = floatingObject.layoutObject();
 
     // FIXME Investigate if this can be removed. crbug.com/370006
     childBox->setMayNeedPaintInvalidation();
 
     LayoutUnit childLogicalLeftMargin = style()->isLeftToRightDirection()
                                             ? marginStartForChild(*childBox)
                                             : marginEndForChild(*childBox);
     if (childBox->style()->clear() & ClearLeft)
@@ skipping 917 matching lines @@
   return LayoutBlock::invalidatePaintIfNeeded(paintInvalidationState);
 }
 
 void LayoutBlockFlow::invalidateDisplayItemClients(
     PaintInvalidationReason invalidationReason) const {
   BlockFlowPaintInvalidator(*this).invalidateDisplayItemClients(
       invalidationReason);
 }
 
 }  // namespace blink