bug#24918: 25.1; Fonts can make Emacs grind to a halt

[Klaus-Dieter Bauer <bauer.klaus.dieter@gmail.com>]

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2016-11-29 18:47 GMT+01:00 Eli Zaretskii <eliz@gnu.org>:

> > From: Klaus-Dieter Bauer <bauer.klaus.dieter@gmail.com>
> > Date: Tue, 29 Nov 2016 11:29:03 +0100
> > Cc: 24918@debbugs.gnu.org
> >
> > The issue occurs with the default fonts too ("emacs -Q"). On Windows
> that means `Courier New' for `default'
> > and `Monospace' for `fixed-pitch', `Monospace Serif` for
> `fixed-pitch-serif' and `Arial` for `variable-pitch'.
> >
> > With these default settings, the `package-list-packages' buffer works
> mostly fine, but the info page `(cl)
> > Structures' for instace does not. Using different fonts, e.g. `Linux
> Libertine Mono' or `Noto Mono', the issue
> > becomes only more widespread.
> >
> > I detail, I noticed that the issue indeed does occur specifically when
> font substitution kicks in. In the info-page
> > for `(cl) Structure' this occurs, because the "=>" is replaced by the
> unicode symbol "⇒", which is displayed in
> > a different font; Sadly I can't figure out a way to identify the
> substituted font; It is definitely a variable-pitch font
> > (the symbol is wider than the default font, such that characters are no
> longer vertically aligned with other
> > lines).
>
> That's very strange, because I see nothing similar on my systems.
>
> As Clément points out, "C-u C-x =" will show the font that is used to
> display those characters.  Please tell what they are; they use Symbola
> here (and that's how things should be, by default).
>


Ah, I missed that line in `C-u C-x ='. (I also don't see any message from
Clément).


#### Using "emacs -Q" ####

It says for the ⇒ (\Rightarrow) symbol
              display: by this font (glyph code)
    uniscribe:-outline-Malgun
Gothic-normal-normal-normal-sans-17-*-*-*-p-*-ksc5601.1987-0 (#x22D)

For the ‖ (\Vert) and ※ (\textreferencemark) symbols:
    uniscribe:-outline-MS
Gothic-normal-normal-normal-mono-17-*-*-*-c-*-gb2312.1980-0 (#x340)

For the  symbol:
    uniscribe:-outline-MS
Gothic-normal-normal-normal-mono-17-*-*-*-c-*-gb2312.1980-0 (#x364)


#### With init file ####

(using Linux Libertine Mono as default font)

The ‖ and ※ symbols work fine in that configuraiton.
"⇒" still causes the issue here too, and falls back to identically the same
`uniscribe' line
    uniscribe:-outline-Malgun
Gothic-normal-normal-normal-sans-17-*-*-*-p-*-ksc5601.1987-0 (#x22D)

The extreme lag in `package-list-packages' seems to be caused by the ▲
symbol in the **header line**, which works fine with the default font, but
is substituted with
    uniscribe:-outline-Malgun
Gothic-normal-normal-normal-sans-16-*-*-*-p-*-gb2312.1980-0 (#x240)
when using `Linux Libertine Mono' or `Noto Mono'.


By any chance, do any of your test systems run Windows and have MS Office
installed? (In my case the 2010 version.) It should be the primary source
of Unicode fonts on my system.

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