Font rendering oddity in status bar

A Debian user reported a cosmetic bug in Ion3 where the top pixel of a
vertical bar character (U+2502) in the status bar could be left behind
when the text changed.  This suggests some sort of error in finding the
text extents (but at what level, I don't know).  The bug report is at
http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=417651 and includes a
screenshot of this behaviour.  I identified the font for most characters
as being fixed 6x13, but the vertical bar appears to be 14 pixels tall;
perhaps it comes from another font in a fontset?  I have spent some time
investigating this (explanations attached to the report) but have not
been able to work out where the mismatch is.  In particular I have not
been able to construct a fontset that includes fixed 6x13.

Can anyone help me work out what's going on?

Ben.

-- 
Ben Hutchings
All extremists should be taken out and shot.

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