Confusing issue with redisplay

[Po Lu]

I've run across an odd issue whilst bringing up a new window system: it
seems that each glyph overlaps its surroundings by exactly one pixel,
which leads to visible artifacting.  For instance, while I would have
expected the output from redisplay to be as follows:

["f" CHAR_GLYPH x=0 width=15 y=0 height=15] ["o" x=16 width=15 y=0
height = 15]

In this case "o" would actually start at 15 on the X axis, causing the
glyph "f" to be partially overdrawn.  What is even more confusing is
that the output of redisplay seems to exhibit the same overlap under
other window systems, whilst no artifacting is visible.

Am I missing something particularly obvious?  Any help would be
appreciated.  Thanks in advance!