A block that establishes a new formatting context is pushed by floats.

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 455 matching lines @@
     LayoutUnit startPosition = borderStart() + paddingStart();
     LayoutUnit initialStartPosition = startPosition;
     if (style()->shouldPlaceBlockDirectionScrollbarOnLogicalLeft())
         startPosition -= verticalScrollbarWidth();
     LayoutUnit totalAvailableLogicalWidth = borderAndPaddingLogicalWidth() + availableLogicalWidth();
 
     LayoutUnit childMarginStart = marginStartForChild(child);
     LayoutUnit newPosition = startPosition + childMarginStart;
 
     LayoutUnit positionToAvoidFloats;
-    if (child.avoidsFloats() && containsFloats() && !flowThreadContainingBlock())
+    if (child.avoidsFloats() && containsFloats())
         positionToAvoidFloats = startOffsetForLine(logicalTopForChild(child), false, logicalHeightForChild(child));
 
     // If the child has an offset from the content edge to avoid floats then use that, otherwise let any negative
     // margin pull it back over the content edge or any positive margin push it out.
     // If the child is being centred then the margin calculated to do that has factored in any offset required to
     // avoid floats, so use it if necessary.
     if (style()->textAlign() == WEBKIT_CENTER || child.style()->marginStartUsing(style()).isAuto())
         newPosition = std::max(newPosition, positionToAvoidFloats + childMarginStart);
     else if (positionToAvoidFloats > initialStartPosition)
         newPosition = std::max(newPosition, positionToAvoidFloats);
@@ skipping 2583 matching lines @@
     FrameView* frameView = document().view();
     LayoutUnit top = (style()->position() == FixedPosition) ? 0 : frameView->scrollOffset().height();
     int visibleHeight = frameView->visibleContentRect(IncludeScrollbars).height();
     if (size().height() < visibleHeight)
         top += (visibleHeight - size().height()) / 2;
     setY(top);
     dialog->setCentered(top);
 }
 
 } // namespace blink