This is going to be a rather short blog post, but I think it’s still worth mentioning. Since 5.26, kwin will support only one way of setting up X screens – Xinerama, multi-head won’t be supported anymore. However, despite how “setup-breaking” it may sound, this will most likely not affect you as you probably already use Xinerama.
Before diving any deeper, it’s worth providing you some background. On X11, there are several ways how you could configure your desktop environment to run with multiple monitors – multi-head and Xinerama.
Multi-head is an old school way to run multiple monitors. Basically, with that mode, there’s an X screen per monitor. In Xinerama mode, there’s only one virtual X screen per all outputs. Both modes have their advantages and disadvantages, for example you can’t freely move windows between screens when using multi-head, etc. Xinerama is younger than multi-head and it provides the most user friendly workflow on multi-screen setups, so it’s usually enabled by default in all Linux distributions and many desktop environments are optimized for running in this mode, including Plasma.
Technically, kwin does provide support for both multi-head and Xinerama. But multi-head support has been in neglected and unmaintained state for many many years, e.g. some code (primarily, old code) supports multi-head, while lots of other code (mostly, new code) does not, various system settings modules and plasmashell components do not support multi-head either, etc. It’s also safe to say that no kwin developer has ever tested multi-head within last 5+ years.
So, rather than keep advertising the support for a feature that we don’t maintain and have no plans to fix it, we decided to drop the support for multi-head mode and make Xinerama a hard requirement since 5.26.
Does this mean that Plasma won’t support multiple monitors anymore?
No, Plasma will continue supporting setups with multiple monitors, but you will need to ensure that Xinerama is used, which is usually the case and you don’t need to tweak anything.
I used multi-head for some esoteric thing, what should I do now?
It’s highly recommended to give the Wayland session a try. If something’s missing, file a bug report or contact us at https://webchat.kde.org/#/room/#kwin:kde.org.