bug#45557: 27.1; Incorrect rendering of COMBINING OVERLINE

[Eli Zaretskii]

> 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.