Don't propagate pagination struts to flow threads.

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 2869 matching lines @@
 {
     // The block needs to be contained by a LayoutBlockFlow (and not by e.g. a flexbox, grid, or a
     // table (the latter being the case for table cell or table caption)). The reason for this
     // limitation is simply that LayoutBlockFlow child layout code is the only place where we pick
     // up the struts and handle them. We handle floats and regular in-flow children, and that's
     // all. We could handle this in other layout modes as well (and even for out-of-flow children),
     // but currently we don't.
     // TODO(mstensho): But we *should*.
     if (isOutOfFlowPositioned())
         return false;
+    if (isLayoutFlowThread()) {
+        // Don't let the strut escape the fragmentation context and get lost.
+        // TODO(mstensho): If we're in a nested fragmentation context, we should ideally convert
+        // and propagate the strut to the outer fragmentation context, so that the inner one is
+        // fully pushed to the next outer fragmentainer, instead of taking up unusable space in the
+        // previous one. But currently we have no mechanism in place to handle this.
+        return false;
+    }
     LayoutBlock* containingBlock = this->containingBlock();
     return containingBlock && containingBlock->isLayoutBlockFlow();
 }
 
 void LayoutBlockFlow::setPaginationStrutPropagatedFromChild(LayoutUnit strut)
 {
     if (!m_rareData) {
         if (!strut)
             return;
         m_rareData = adoptPtr(new LayoutBlockFlowRareData(this));
@@ skipping 166 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