bug#9366: Display geometry change hook

[martin rudalics]

>>   > Would it work to test the display geometry from focus-in-hook?
>>
>> Sounds expensive
>
> How so?  Is the API which we use to determine the display geometry
> expensive?

I meant expensive in the sense that Emacs would poll the display
geometry in every focus event with maybe a handful of people ever making
use of it.

> Any quantitative data about that?
>
>> And what about a user who currently has no frame on that display but
>> might consider switching to it when its geometry meets certain
>> requirements?
>
> Is this an important use case?

I don't know.  I never plug a monitor into a running session.

>>   > Or
>>   > from a timer?
>>
>> To poll the geometry?  Sounds much too expensive too.
>
> Same question as above.  We currently have several timers running in
> every session, so a timer that ticks, say, once a second doesn't sound
> too expensive to me.  Especially since this will most probably be an
> optional feature.

You mean when the value of 'display-geometry-change-hook' is non-nil,
for example.

>> If and when the underlying windowing system informs us of display
>> changes, I would just react to them.  What's speaking against it?
>
> The proposed solution was only for X, and using an optional component
> at that.  I'd rather find a solution that would work on all supported
> platforms and required no special APIs.

But it would probably rely on 'display-monitor-attributes-list' and thus
use its APIs.  And on Windows, for example, the "special" API is already
there in WM_DISPLAYCHANGE and I suppose the other platforms should be
able to handle fullscreen frames after a display change in a similar way
too.

In either case it's no great deal.  I won't implement it because I've
sworn to never change the resolution of a running system again (once an
xfce Debian here insisted for weeks to come up with a 640x480 screen
resolution because it did not trust my monitor's repeat frequency) and
master still struggles with the (funcall eldoc-documentation-function)
issue.

martin