bug#22519: 25.1.50; Emacs gets stuck while doing incremental search forward

[Eli Zaretskii]

> From: vincent.belaiche@gmail.com (Vincent Belaïche)
> Cc: Vincent Belaïche <vincent.belaiche@gmail.com>,
>   22519@debbugs.gnu.org
> Date: Thu, 17 Mar 2016 10:14:04 +0100
> 
> - it would suffice to raise some character specific flag when a
>   character is not found in any font in ordder to prevent subsequent
>   search for the same character after an edit, and also to put this
>   character in some queue so as to trigger the actual font search
>   _again_ for the same character only when Emacs gets the focus.

The fonts are searched when Emacs is about to redisplay the window.
Whether Emacs has the focus at that time is not really relevant, since
if there's a need to redisplay, Emacs must do that regardless.

I don't quite understand your proposal about "preventing subsequent
search for the same character after an edit": installation of
additional fonts is unrelated to anything don inside Emacs, so whether
there was or wasn't any editing doesn't seem relevant.

> - there could be some counter incremented every time a character cannot
>   be displayed in a known font and alternative font would be extensively
>   searched only when this counter is below some threshold, and this
>   counter would be periodically reset. This method has however the
>   drawback that the display would not be deterministic.

I think we already do something like that, but I'm far from being an
expert on this part in Emacs.

> - I am wondering whether it is to the application to make this search,
>   the system is supposed to know which fonts have been installed, isn't
>   it possible that Emacs just ask to MSW for each character that cannot
>   be displayed in the default font to supply the best alternative
>   font. Then, since the OS is supposed to know whether or not a new font
>   has been installed it may be more efficient in performing this search.

The OS knows about installed fonts, but I'm not sure there's a service
to announce that to applications.  The OS certainly doesn't know
whether the installed font supports some character; it provides APIs
for applications to find that out, which is what Emacs does.

> Would it be agreable if I make the following addition to the manual:

I think this should be rewritten to go to etc/PROBLEMS instead of into
the manual.

Thanks.