The fix in 6d2e09db is completely bogus.
Only vertically merged boxes result in dummy boxes with negative span,
while horizontally merged boxes result in different numbers of boxes per
line.  So instead of inserting boxes, adjust the width of the last box
in rows that are missing boxes, such that all lines have the same width.

Change-Id: I4e90e852b314bf6f7885bde7b450dab7c668469e