Clear floats that are nested in previous siblings and don't overhang.

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 411 matching lines @@
     updateLogicalHeight();
     LayoutUnit newHeight = logicalHeight();
     if (!childrenInline()) {
         LayoutBlockFlow* lowestBlock = nullptr;
         bool addedOverhangingFloats = false;
         // One of our children's floats may have become an overhanging float for us.
         for (LayoutObject* child = lastChild(); child; child = child->previousSibling()) {
             // TODO(robhogan): We should exclude blocks that create formatting contexts, not just out of flow or floating blocks.
             if (child->isLayoutBlockFlow() && !child->isFloatingOrOutOfFlowPositioned()) {
                 LayoutBlockFlow* block = toLayoutBlockFlow(child);
+                if (!block->containsFloats())
+                    continue;
                 lowestBlock = block;
                 if (oldHeight <= newHeight || block->lowestFloatLogicalBottom() + block->logicalTop() <= newHeight)
                     break;
                 addOverhangingFloats(block, false);
                 addedOverhangingFloats = true;
             }
         }
+        // If we have no overhanging floats we still pass a record of the lowest non-overhanging float up the tree so we can enclose it if
+        // we are a formatting context and allow siblings to avoid it if they have negative margin and find themselves in its vicinity.
         if (!addedOverhangingFloats)
             addLowestFloatFromChildren(lowestBlock);
     }
 
     bool heightChanged = (previousHeight != newHeight);
     if (heightChanged)
         relayoutChildren = true;
 
     layoutPositionedObjects(relayoutChildren || isDocumentElement(), oldLeft != logicalLeft() ? ForcedLayoutAfterContainingBlockMoved : DefaultLayout);
 
@@ skipping 2637 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