bug#45557: 27.1; Incorrect rendering of COMBINING OVERLINE

[Stephen Eglen <sje30@cam.ac.uk>]

On Thu, Dec 31 2020, Eli Zaretskii wrote:

>> From: Stephen Eglen <sje30@cam.ac.uk>
>> Cc: Stephen Eglen <sje30@cam.ac.uk>, 45557@debbugs.gnu.org
>> Date: Thu, 31 Dec 2020 14:12:25 +0000
>> 
>> > If you move the cursor to x̅, does Emacs display a single cursor block
>> > that includes both x and the overline, or does it behave as if those
>> > were 2 separate glyphs?  If the latter, what does Emacs show in the
>> > *Help* buffer if you go to the ̅ glyph and type "C-u C-x ="?
>> 
>> It appears as two separate glyphs.  The first is char "x" (decimal 120)
>> and then the second is given as below
>> 
>> position: 86 of 87 (98%), column: 3
>>             character: ̅ (displayed as ̅) (codepoint 773, #o1405, #x305)
>>               charset: unicode-bmp (Unicode Basic Multilingual Plane (U+0000..U+FFFF))
>> code point in charset: 0x0305
>>                script: latin
>>                syntax: w 	which means: word
>>              category: ^:Combining
>>              to input: type "C-x 8 RET 305" or "C-x 8 RET COMBINING OVERLINE"
>>           buffer code: #xCC #x85
>>             file code: #xCC #x85 (encoded by coding system utf-8-unix)
>>               display: by this font (glyph code)
>>     ftcrhb:-UKWN-JuliaMono-normal-normal-normal-*-19-*-*-*-m-0-iso10646-1 (#x1F1D)
>
> And the same info about "x" also shows the same font, i.e.
>
>   ftcrhb:-UKWN-JuliaMono-normal-normal-normal-*-19-*-*-*-m-0-iso10646-1
>
> Anyway, the above means Emacs didn't compose these COMBINING OVERLINE
> woth "x", for some reason.  The question is why.

I think this can be closed now!

After much debugging and reinstalls of Emacs under different
configurations, I found the problem is not with Emacs, but the way in
which I installed JuliaMono.

If I install the fonts using

yay -S ttf-JuliaMono

i.e. using the arch package manager, I get the problem.  If I uninstall
that package, and install the fonts manually, by putting them in my
~.fonts/ folder and running "fc-cache -fv", the combining characters
work fine.

I could not find out why this made any difference.  I can see one small
issue, i.e. that when installing fonts using the package manager, it
does a little bit of extra work, by running some hooks.  These hooks
effectively run "mkfontscale" and "mkfontdir" in the system directory,
and update the files /usr/share/fonts/TTF/fonts.dir,fonts.scale .
However, if I temporarily removed those hooks, I still get the problem.

Either way, this is much an arch problem not an Emacs problem I feel.

Thanks everyone for your help.

Stephen