Only show interactive lock screen controls on the primary screen
Currently, when using a multi-screen set up, an identical copy of the lock screen UI is displayed on all screens.
Various people in the VDG have desired to show the lock screen only on one screen, in the case when there are more than one. Other screens would just show a black background, or a static blurred copy of the wallpaper, or whatever. VDG folks can figure it out. :)
The primary reasons for this would be usability and aesthetics. Usability is improved anytime we let the user avoid making a choice, so only showing one lock screen would make it obvious that you interact with the single thing that's visible. In terms of aesthetics, it would be nicer to not duplicate the same interactive UI on multiple screens, which is a bit visually awkward right now.
A secondary benefit would be de-complexification of the implementation. Having to handle multiple screens means multiple sources of data signals that can cause race conditions. The recent lock screen PAM work for Plasma 5.25 generated several of these that range from annoying to severe. If we could guarantee only one interactive lock screen UI on one monitor, all the code that handles multi-screen complexities can be removed, and bugs due to race conditions from showing multiple UIs are fixed automatically.
In the past I've been told that we had to show the interactive UI on all screens for technical reasons: at the time there was no Primary Screen concept on Wayland, so the screen locker couldn't guarantee that the interactive UI would be shown on an enabled screen, in the case when some were turned off and it had to choose a single screen to appear on. Also without a Primary Screen, the actual screen it appeared on could vary over time seem random to the user.
But today our Wayland session does have a Primary Screen. So the screen locker can safely target the Primary Screen for both the Wayland and X11 sessions to show the interactive UI on, and all other screens can be left blank.