Add scale configuration when under wayland

[stefonarch]

Expected Behavior

Also scale should be configurable under kwin_wayland

Current Behavior

It's not.

Context

Different scale settings for different monitors can be useful in LXQt Kwin_wayland

[stefonarch]

If monitors are scaled (using systemsettings) lxqt-config-monitor can't handle it:
b.com//issues/996, and atm only KDE can configure scaling.

In this case at next login it will apply its configuration which is

and the monitor setup will be completely broken (cursor can't exit primary monitor) until it's fixed by KDE's systemsettings (where it sees a space between monitors which is invalid).

[tsujan]

Theoretically, we just need to add KScreen::Output::scale and KScreen::Output::setScale to the GUI.

[stefonarch]

And the previews of the monitors will be fine too in their size?

[tsujan]

Making previews work with scaling is only one part of the job. We need to add GUI elements to set and get the scales of all monitors, depending on whether we're on Wayland or not. We also need to save the new settings in a backward compatible way.

And these steps are just what came to my mind without writing a single line of code — sometimes there's a long way from theory to practice ;) But it's good to know that there is a way.

[stefonarch]

Besides scaling also "This is my primary monitor" should be changed under kwin_wayland.