[Development] Wayland: getting a screen's fractional scale before the first window exists

[Henry Skoglund]

On 2026-08-24 18:27, Fatih Uzunoğlu wrote:
> Hello,
>
> Regarding:
>
> > until the surface is configured by the compositor
>
> I am not sure if the protocol guarantees when 
> `wl_surface::preferred_buffer_scale` is sent (or 
> `wp_fractional_scale_v1::preferred_scale` in fractional scale case), 
> but if in practice major compositors send the scale within one or two 
> event loop cycles, then for the sake of application's convenience 
> maybe `QWindow::create()` may do one or two arbitrary 
> `wl_display_roundtrip()` after the initial commit just to expect the 
> compositor to send the scale.
>
> The waiting time here would probably be negligible, and in turn the 
> application can immediately use the proper scale since the roundtrip 
> call is blocking, provided that the compositor sent it.
>

That's pretty good advice for any Qt application I think.
I only use QWidgets apps, but for those I do a QTimer::singleshot(20 
milliseconds) from MainWindow's constructor. Then in the timer code I 
can reliable get the screen geometry/geometries (instead of the 640x480 
when inside the ctor).

Rgrds Henry
P.S. Also easier to autostart or quit the app then, since QApplication's 
event loop starts efter MainWindow's ctor.