Never move unbreakable blocks that are at the top of a column / page.

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 836 matching lines @@
     bool isUnsplittable = child.isUnsplittableForPagination() || (checkColumnBreaks && child.style()->columnBreakInside() == PBAVOID)
         || (checkPageBreaks && child.style()->pageBreakInside() == PBAVOID);
     if (!isUnsplittable)
         return logicalOffset;
     LayoutUnit childLogicalHeight = logicalHeightForChild(child) + (includeMargins ? marginBeforeForChild(child) + marginAfterForChild(child) : LayoutUnit());
     LayoutUnit pageLogicalHeight = pageLogicalHeightForOffset(logicalOffset);
     updateMinimumPageHeight(logicalOffset, childLogicalHeight);
     if (!pageLogicalHeight)
         return logicalOffset;
     LayoutUnit remainingLogicalHeight = pageRemainingLogicalHeightForOffset(logicalOffset, AssociateWithLatterPage);
-    if (remainingLogicalHeight < childLogicalHeight)
+    // Break if there's not enough space left for us, but only as long as we're not already at the
+    // top of a page. No point in leaving a page completely blank.
+    if (remainingLogicalHeight < childLogicalHeight && remainingLogicalHeight < pageLogicalHeight)
         return logicalOffset + remainingLogicalHeight;
     return logicalOffset;
 }
 
 void LayoutBlockFlow::rebuildFloatsFromIntruding()
 {
     if (m_floatingObjects)
         m_floatingObjects->setHorizontalWritingMode(isHorizontalWritingMode());
 
     HashSet<LayoutBox*> oldIntrudingFloatSet;
@@ skipping 2199 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