bug#18195: 24.3.92; window-screen-lines is not accurate

[Eli Zaretskii <eliz@gnu.org>]

> Date: Wed, 06 Aug 2014 16:09:41 +0400
> From: Dmitry Gutov <dgutov@yandex.ru>
> CC: rudalics@gmx.at, 18195@debbugs.gnu.org
> 
> On 08/06/2014 06:52 AM, Eli Zaretskii wrote:
> 
> > Until some user would ask to lift them.
> 
> Apparently, every user that tried to change the font understood that any 
> resulting problems stemmed from their actions, and so we've never had 
> any bug reports about that.

AFAIU, you are developing infrastructure.  It is generally expected
from infrastructure, at least that in Emacs core, to support
everything that wasn't explicitly documented as forbidden or
unsupported.

OTOH, perhaps next time someone complains about some weird combination
of display features that happen not to work, I should respond
"restriction" and see what happens.

> > But if you never want to lift them, why not define colors instead of
> > full-fledged faces?  Then those restrictions are really carved in
> > stone and clear for all.
> 
> Well, there are also text decorations, and users should be able to add 
> (most of) them without running into problems.

Which decorations are those?

> Finally, who's to say the current limitations will stay with us forever? 
> If Emacs defines a suitable API, the current faces should be able to 
> transition seamlessly, while users will also gain the ability to change 
> the fonts.

Nothing like that will happen in Emacs without motivated individuals
driving the development.  IMO, we should either push development in
the direction I suggested (or similar), or explicitly forbid using
different fonts in Company completion, at least in the documentation,
if not by error messages.  Anything else is unwise, IMO.