bug#26742: Display bug with composed strings

[Eli Zaretskii <eliz@gnu.org>]

> Cc: 26742@debbugs.gnu.org
> From: Clément Pit--Claudel <clement.pitclaudel@live.com>
> Date: Tue, 2 May 2017 13:22:19 -0400
> 
> For ℝ, "emacs -Q" uses "Latin Modern Math" and "emacs" with customizations uses "XITS Math". 
> For ≤, both use Ubuntu Mono.  I changed the default font and noticed slightly different glitches, including with a variable-pitch font multiple "!" disappearing.

Could you try fonts mentioned by Andreas?

> > If that doesn't help, then the next question is: is this problem 
> > specific to these two characters, or is it more general?
> 
> More investigation suggests that the problem only happens when both parts of the composition are not displayed with the same font.  For example, if I set my default font to Latin Modern Math, the problem goes away.

Does the problem affect other characters as well, or just these two?

> > If the latter, I suspect the shaping engine used in your case is the
> > culprit, so perhaps upgrade your libotf, libm17n-flt, and m17n-db
> > packages.  
> 
> Apt tells me these packages are up-to-date, but it might not be offering me the very-latest version:
> 
> $ apt show libotf0
> Package: libotf0
> Version: 0.9.13-3
> 
> $ apt show libm17n-0
> Package: libm17n-0
> Version: 1.7.0-3
> 
> $ apt show m17n-db
> Package: m17n-db
> Version: 1.7.0-2

Time to ask Handa-san to please chime in and comment on this issue,
and first of all a question: Is this feature supposed to work when the
characters are displayed not by the same font?  It seems to work with
the Uniscribe shaping engine on Windows, but not with libotf/libm17n.

Thanks.