I just upgraded from CentOS 7 (KDE 4.x) to CentOS 8 (KDE 5.x) and it looks like there is a nasty regression impacting users who use multi-input very high end monitors such as:

IBM T221
Dell UP2715K
Dell UP3218K

and others.

Specifically, even though randr understands the concept of composited monitors:

Code: Select all

$ xrandr --listmonitors
Monitors: 1
 0: +Auto-Monitor-4c454440b630343253 5120/600x2880/340+0+0  DP-2 DP-4


the workspace still insists on presenting them as two separate monitors and there is no way to stretch the desktop and the panel seamlessly across both halves of the monitors.

The workaround in older versions was to use Nvidia Mosaic capability (NoTwinViewXineramaInfo=True / nvidiaXineramaInfo=False in xorg.conf), which made it fake Xinerama geometry to the true geometry of the monitor, but Xinerama and RandR are both GDM and KDE 5.x seem to require RandR and are incompatible with Xinerama (black screen).

So it seems that KDE 5.x both required RandR _and_ doesn't understand enough of the RandR feature set to make this work.

Is there a fix/workaround for this? How do I stretch my desktop properly across the entire Dell UP2715K?

I want to push it up again.

Until now, I used the workaround by configurating the Nvidia driver to use the "xinerama" option, the same as gordan mentioned. This led to some confusion within KDE in respect to icon sizes and display configurability, but at least it worked. With my yesterday's update to Kubuntu 22.04 (with the software versions below, copied from info center), the cog rotates half a minute and then I get a black screen. I see a cursor, but nothing else. I can log out by pressing the power button. It seems that somewhere KDE crashes, but I don't know where to look for any kind of error reports. I am not an expert in the Linux display infrastructure and KDE desktop, and I don't know how to solve this problem. I once picked up a developer from the kscreen component (as far as I remember), but he could not or did not want to help.

This is totally frustrating. I once tried Cinnamon desktop and it worked like a charm. But I do not like the Gnome look & feel that much and clearly prefer KDE.

Operating System: Kubuntu 22.04
KDE Plasma Version: 5.24.4
KDE Frameworks Version: 5.92.0
Qt Version: 5.15.3
Kernel Version: 5.15.0-25-generic (64-bit)
Graphics Platform: X11
Processors: 12 × Intel® Core™ i7-5930K CPU @ 3.50GHz
Memory: 31,3 GiB of RAM
Graphics Processor: NVIDIA GeForce GTX 980 Ti/PCIe/SSE2

Just an update. I did some further research on this topic and my current guess is that the main source of the trouble is Qt rather than KDE. Deep in the code of Qt, there is a class named QXcbConnection (https://github.com/qt/qtbase/blob/dev/s ... creens.cpp) which interacts with the X server, especially with xrandr. It seems to be responsible to discover the "screens", and the Qt version 5.15.3 regards each output as a separate screen, which causes my and others' problem.

Fortunately, the Qt team seems to be in the process of reworking the code. In the current development branch, the code checks the version of the xrandr extension and if it is new enough, it won't rely on the outputs, but on the monitor definitions supplied by xrandr. This gives some hope that the problem will be solved in a future version, whenever this code becomes part of it (KDE6?).