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

[Klaus-Dieter Bauer]

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.