Width of fringes

[Lele Gaifax <lele@metapensiero.it>]

Hi all,

a bit of context first: just to learn something new I'm playing with the
fringe, writing my own custom minor mode to show the test coverage of each
line in a buffer; I used three different fringe symbols 'filled-rectangle,
'hollow-rectangle and 'question-mark to show respectively "executed", "missed"
and "excluded" lines of code.

Everything is going smooth, but I have a "visual glitch" that I was not able
to understand: while with standard settings (ie "emacs -Q") all three symbols
appear "complete", using my own configuration they appear "truncated" on the
left, in other words for example the 'hollow-rectangle comes out as

  xxxxxx.
  .....x.
  .....x.
  .....x.
  .....x.
  .....x.
  .....x.
  .....x.
  .....x.
  .....x.
  .....x.
  .....x.
  xxxxxx.

Looking closely, I see that the right fringe is actually a tiny bit wider that
the left one, and indeed if I put those symbols in the right fringe they come
out "complete".

I obviously checked my configuration, but nothing in it explicitly changes the
fringe-styles or whatever, and the global fringe-mode is nil, and both
left-fringe-width and right-fringe-width are nil too.

Does anybody have an hint on what could cause the different width of the two
fringes?

FWIW, I'm using Emacs master, compiled a couple of days ago, on a Debian sid.

Thanks in advance,
stay safe, ciao, lele.
-- 
nickname: Lele Gaifax | Quando vivrò di quello che ho pensato ieri
real: Emanuele Gaifas | comincerò ad aver paura di chi mi copia.
lele@metapensiero.it  |                 -- Fortunato Depero, 1929.