I don't know if this is the correct forum to ask (if not please advise me where to go), but here it goes:

I recently changed my monitor to a newer model. Since the change KDE cannot remember the settings of which screen is the primary and its size. I have to go into system settings->Display and monitor in order to setup everything after a boot.

Does this behaviour has something to do with kwin? If so how can I fix the problem?

Running Fedora 30 KDE spin on a X230 laptop with a ThinkVision attached through DVI->HDMI adapter.

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I have the same or very similar problem. I have two identical monitors, plugged into the two DVI ports of a "1c:00.0 VGA compatible controller [0300]: Advanced Micro Devices, Inc. [AMD/ATI] Cape Verde XT [Radeon HD 7770/8760 / R7 250X] [1002:683d]" video card. Everything was stable until, while doing other work behind the tower, I switched the two cables. Now when I start KDE, the "preferred" screen shows on the left monitor, and the mouse runs off the right edge of the right monitor and then shows up on the left edge of the left monitor. If I run systemsettings5, display dialog, drag the screens to the other order, and click "Apply," things go back to the way I want them, and the mouse moves properly across the two monitors. The problem is that if I logout (actually just exit X, since I don't use a DM) and restart, I have to do it again. I have tried with both "Save display's properties:" as either "For any display arrangemtn" and "For only this specific display arrangement" with no difference. I don't know if it matters or not, but even if I click "Primary" on the right monitor, it is unchecked the next time I start Plasma. I do not have xorg.conf, and there are no files in /etc/X11/xorg.conf.d.

I have kwin 5.17.5, but I have had this problem across several versions of plasma. I know I could just swap the cables again, and I'd rather not have to create an xorg.conf - but am I wrong in thinking this should be remembered across plasma sessions?

Did you try saving the plasma session on logout? I have two monitors conntected via DVI and HDMI since several years and have never run into such a problem.

If it is set to restore previous session (which it was) I thought it should automatically save the session. However, I also tried restore manually saved session, and explicitly saved the session, but still the same behavior on restart. However, it didn't give me any option of choosing which saved session to restore - unless it only saves one.
It seems to me the monitor layout is not being saved as part of the session. Do you know where the saved session is stored, so I might take a look and see if the display/monitor information is included?

Indeed, the monitor settings are not saved, just checked and couldn't find a place where that is actually stored, at least not in the users home files. No idea where the monitor settings are stored, though.

I found this thread on Reddit: https://www.reddit.com/r/kde/comments/b ... er_reboot/

where an entry in /etc/X11/xorg.conf solved the problem. I never had to do this, but maybe some monitor setups need specific setups, even now...

In any case, this should just not happen, please make sure to file a bug at https://bugs.kde.org for the component "multiscreen support" in plasmashell

Thanks for the info on the right component to tag. I filed https://bugs.kde.org/show_bug.cgi?id=420265

I know an xorg.conf entry could fix this, and so could swapping the cables again, but if plasma should remember the positions, it should remember. If it turns out that's not the case, I'll switch my bug report to a wishlist so that the positions ARE saved.