Ah, working with more than one screen, nice and convenient, but always has been the scourge of usability, due to broken hardware and software, on every platform you can imagine. And yes, on Plasma we are notorious to have several problems with regard to multi screens, which spurred many bug reports.
TL;DR
For Plasma 5.27 we did a big refactor on how the screens are managed (and how they are mapped to desktops and panels) which hopefully should solve many of those issues, creating a much more predictable experience, both for “fixed” multi screen setups (like a setup at your desk) and “on the go” (like attaching a projector at a conference).
- No more default empty desktops after connecting a screen that you already connected in the past
- No more desktops, wallpapers, widgets and panels lost after restart
- No more different sets of desktops between X11 and Wayland
- Better experience when using USB-C based docks
UI wise on the Systemsettings module nothing will change in the most common case, when you have one or two screens, where you will be presented with the usual “primary” checkbox.
When you have 3 or more screens, instead of s simple “Primary” checkbox, you will be presented with a dialog where you can rearrange the screens in a logical order, so that you can say: “This is my screen number one, this is my screen number two etc.”
This will map exactly to what “screen number one, two etc” are for the Plasma shell, so that whatever desktops, wallpapers, widgets and panels are on the “screen number one” will always be on the “screen number one”, never disappearing again, same for each screen, giving a completely predictable, and stable multiscreen setup.
For 5.27 I worked mostly on the Plasmashell part, but many thanks and shouts to Ivan for his work on KScreen and to Xaver for his work on the KWin and Wayland protocol parts.
Long story: the moving parts
I would like to do a semi-technical but still quite high level description on how our multiscreen infrastructure works, and what did we change.
When you have multiple monitors connected, assuming the kernel and drivers will do the right thing (which… doesn’t always happen), either X11 or Wayland (so in our case the KWin Wayland compositor) will see the screens and have info about them we can enumerate and manage.
KScreen
In Plasma we have a daemon/library to manage the screens, called KScreen. It will save a configuration for each combination of screens it encountered. each screen is identified uniquely by its EDID, and when this fails for the above reasons, the connector name the screen is connected to (under X or libdrm they will have names like HDMI-A-1, DP-2, eDp1 and so forth). connector names are also not optimal, but is the best that can be done in case of a bad monitor (For 5.27 Xaver fixed a quite big issue on this fallback path which likely caused several of the reported bugs).
The user then can chose with the Systemsetting module how the screens are arranged, their resolutions, which is the primary and what not. When a screen is connected or disconnected, KScreen will search for an existing configuration for this set of screens and restore that one.
Plasma
The Plasma shell also has to be aware of multiple screens arrangement, because you want the desktops with their wallpapers, widgets and icons to be on the screen you expect it to, as well all the panels to be always on the “proper” screen you expect it to. And that’s where we had a lot of problems in the past, and various approaches have been attempted.
Current
Currently, up to 5.26 Plasma relied on two concepts for working out which screen a desktop and panel should be. Combining a single primary monitor and using screen connectors names for disambiguate second and third screens. In theory …
There are a couple of things that can go wrong with this: between X11 and Wayland, connector names tend to be different (for instance HDMI-1 vs HDMI-A-1 on my system).
Another problem is related to the the USB-C based docks that have video outputs (such as the Steamdeck one, but is common to any other model and brand). They dynamically create and delete the outputs and connectors when they are plugged and unplugged, but the connector name is not stable between different plugs/unplugs: sometimes after re-plugging the old names are not recycled, but increased (so for instance if those were DP-2 and DP-3, they may become all the sudden DP-4 and DP-5). Since we used to store those names, we have a problem: each time a new connector name is encountered is treated as a completely new thing, giving its new empty default desktop and no panels, causing many wtf moments.
Plasma Containments (technical name of the desktops and panels) always had a screen property associated to them which is just an integer: 0 for the primary screen and then 1,2,etc for the others.
5.27
The new approach builds on a single design, extending the primary monitor concept to multiple monitors, which cleans up the codepaths even for the two monitor case.
So we now dropped this association map between screen number integers and connector names, and we go with this ordered list all the way down, so now Plasma, KWin and KScreen agree exactly who “Screen 1” is.
We have a new Wayland protocol for screen ordering (as well an X11 counterpart) for KScreen and KWin to agree to which one is the real logical ordering of the screens, so Plasmashell will now follow this without attempting to identify monitors by itself, removing a very big failure point.
As said, When your system has only 2 screens, in the Systemsettings module you’ll see exactly the UI it had: just a checkbox to select which one is the “Primary”, so no surprises here. When you have 3 screens or more then you are presented with a dialog where you can visually reorder the screens, to decide which is the first, which is the second etc.
In this way, you’ll always be sure that whichever screen was set as “first” will have your “main” set of panels and desktop widgets, that you will never lose, also whichever you set as “second” will always have the desktop and panels you set it and never change and so on.