Help me debug a strange multi monitor problem

[DromundKaas]

Good morning,

I’m having this problem for as long as I can remember, and I can work around it, but it always leads to certain strange phenomena, and I’d like to know what I am doing wrong, or which piece of software is in error.

My setup: KDE Plasma with 2 monitors of the same type, iyama G-Master GB2788HS, connected to two Displayports on an NVidia Geforce 3070. The problem exists in the multi monitor space.

When I open the display settings in the plasma options, it presents itself this way:

You can see both monitors, and to keep them apart the settings calls the two monitors DP-0 and DP-2 (probably Displayport 0 and 2). Just to clarify, my card has no other Displayports, the only other connector is an HDMI which has no monitor attached.

Using the button “Identify” (Identifizieren in my german language), it displays the names of the two monitors as an overlay. And here the problem shows itself: Although the left monitor is “Primary” (Primär) and has the bottom KDE bar, the identification specifies that the left monitor is DP-2, while in the displays in the settings it is DP-0. So somehow there’s a disconnect between what the detection of the displays says and what I can see from the connections and the graphical display in the settings.

This leads to strange things: No matter how I connect the two monitors to the graphics card, whenever I configure Plasma so that the left monitor has the bottom bar and the mouse and display extend to the right monitor, the - lets say default - display seems to be always the right. When I boot, everything always starts on the right monitor, while the left monitor is Primary (as in the settings), all the games start on the right monitor.

I can switch the connection to the graphics card around, then configure it again so that the left monitor is Primary, extends to the right, and the problem stays the same.

[DromundKaas]

I’ve fiddled around with nvidia-settings a bit in the hope that the problem might be fixed there, but no such luck. I switched the cables again, and DP0 is now the left monitor (which is great), but as soon as I make the monitor identified as DP-0 primary, the bottom bar is displayed on DP-2, and vice versa.

[DromundKaas]

What triggered my newly found interest in analyzing and maybe fixing the problem was that Cyberpunk 2077 currently crashes for me, and I’ve found reports that it could be a problem of having a second monitor on NVidia hardware. So I just tested something, and the result looks a bit like from clowns world.

Disconnecting any of the two cables to the display ports disabled either of the two displays. Yet no matter which one I disconnect, neither has a bottom bar when the other one is disconnected.

Oh, and Cyberpunk crashes the same with only one monitor.

[petsam]

It seems you are experiencing the well known KDE multi-monitor nightmare.

I suggest to first check KDE bug tracker for a similar reported issue and try to add your info/experience.
Although they have done much improvement on the matter, the complexity of their code approach, the complexity of KDE super many features and the complexity of the hardware possible scenarios does not clear the sky.

Some things might help you during investigation:

• Check the panel configuration tool, which may show lost monitors, with panels and monitor names
• Check Xorg log and xrandr output for port and monitor names
• Check journal for related info

If you want more advice, logs are required, with good description of the time it was taken and the status of monitor cable connections. System info is obviously needed.

[DromundKaas]

Thanks for the info. That’s a really nice rabbit hole to get into. Maybe I’ll try Gnome for fun (and profit). And so far, the workaround is acceptable. I’ll see if I can open a new issue, or add my info to an existing one.

Cheers.

Edit: I’d really never have guessed that 2 monitors of the same make with the same resolution and frequency attached by the same connector could make such a strange mess.

[DromundKaas]

I tested out some other DEs yesterday, from Gnome current to XFCE to Budgie, and none of the other ones had the problem described. And on a whim, I installed fresh with Budgie. So the problem is not current anymore. Thanks for you help.