This patch fixes things for me:

https://invent.kde.org/genpfault/plasma-workspace/commit/181fc5746e9b4aaf491a90e76e770df866d880ad

Thanks! Can you please submit that as a merge request? See https://community.kde.org/Infrastructure/GitLab

A possibly relevant merge request was started @ https://invent.kde.org/plasma/plasma-workspace/-/merge_requests/352

Seems related:
https://bugs.kde.org/show_bug.cgi?id=416739

*** Bug 416739 has been marked as a duplicate of this bug. ***

*** Bug 425897 has been marked as a duplicate of this bug. ***

Same issue, I believe. Would love to see this fixed!

After a couple of months of tranquility, the issue suddenly reappeared a month ago, with a vengeance :-(

And now it's stuff on *both* monitors that goes wonky.

- Task managers set up to show only tasks from current screen suddenly show stuff from other screen, and only stuff from the other screen (i.e. the task manager on left hand screen shows tasks from right hand screen, and the task manager on right hand screen shows stuff from left hand screen)
- Spacers disappear (i.e. systray and clock move flush left against whatever stuff is on the left hand side of the panel, rather than staying at the right border)
- Occasionally panels disappear altogether (or move to hide under the equivalent panel on the other screen)
- Clock widgets configured to show seconds suddenly no longer show seconds
- Full screen applications move to the other screen

Time to bring out that handy restartPlasmaShell script again, and add a shortcut to it to the panel:

#!/bin/sh

killall plasmashell
nohup plasmashell >/dev/null 2>&1 </dev/null &


Ironically enough, occasionally that shortcut disappears to :-)

(Just adding myself to the Ccs. Same issue, up-to-date kubuntu 21.10, installed yesterday. Seems related to external monitor and doesn't reproducibly happen with each restart. Will evaluate the workarounds outlined here and see what happens...)

Wondering, though, whether there's a more reliable workaround for that. Restarting plasma seems to work just sometimes, but sometimes the panel just won't re-appear or apparently there's no way of making it show up again, leaving adding a new panel (and reconfiguring everything inside) the only option. That's pretty tedious if one has a customized panel. :(

(In reply to Kristian Rink from comment #10)
> Wondering, though, whether there's a more reliable workaround for that.
> Restarting plasma seems to work just sometimes, but sometimes the panel just
> won't re-appear or apparently there's no way of making it show up again,
> leaving adding a new panel (and reconfiguring everything inside) the only
> option. That's pretty tedious if one has a customized panel. :(

After reconfiguring everything as I need it, I store a backup copy of plasma-org.kde.plasma.desktop-appletsrc and plasmashellrc, and the restore those in my restart script:


#!/bin/sh

#kquitapp plasmashell
killall plasmashell
cp -a $HOME/.config/PlasmaBackup/* $HOME/.config/
nohup plasmashell >/dev/null 2>&1 </dev/null &


Of course, after performing a *wanted* change, I have to copy the files over to the reference again.

(In reply to Alain Knaff from comment #11)
> (In reply to Kristian Rink from comment #10)
> > Wondering, though, whether there's a more reliable workaround for that.
> > Restarting plasma seems to work just sometimes, but sometimes the panel just
> > won't re-appear or apparently there's no way of making it show up again,
> > leaving adding a new panel (and reconfiguring everything inside) the only
> > option. That's pretty tedious if one has a customized panel. :(
> 
> After reconfiguring everything as I need it, I store a backup copy of
> plasma-org.kde.plasma.desktop-appletsrc and plasmashellrc, and the restore
> those in my restart script:
> 
> 
> #!/bin/sh
> 
> #kquitapp plasmashell
> killall plasmashell
> cp -a $HOME/.config/PlasmaBackup/* $HOME/.config/
> nohup plasmashell >/dev/null 2>&1 </dev/null &
> 
> 
> Of course, after performing a *wanted* change, I have to copy the files over
> to the reference again.

Ah cool, thanks, that should do for my purposes too. Not changing desktop too often once it has been set up anyway. :)

Same issue here, and it occurs randomly every now and then... The workaround I've found so far is to:
- kquitapp5 plasmashell
- Edit ~/.config/plasmashellrc and remove all entries under [ScreenConnectors]
- kstart5 plasmashell && kquitapp5 plasmashell
- Read ~/.config/plasmashellrc [ScreenConnectors] to get new screen numbers
- Edit ~/.config/plasma-org.kde.plasma.desktop-appletsrc and manually set lastScreen values for Containments as needed.
- kstart5 plasmashell

Maybe my issue is connected to this one:
I have two screens and use a KVM to switch between two PCs. I can consistently reproduce the problem by switching the PCs. Every time I switch from my main PC to my work-laptop and back, my second screen has lost its panel and configurations. If I switch to my laptop and PC AGAIN, everything is back the way it was. So every uneven switch, I lose my settings and every even switch I get them back.
I'm using KDE Plasma 5.23.5 with KDE neon 20.04.

Same problem here with plasma-worskpace-5.18.5-0ubuntu0.1
I have two monitors (occasionally adding a hdmi TV with another graphic card). My panel is vertical on the left of the left monitor.

The problem occurs randomly after the monitors went to powersave mode (or manually turned off). 

Maybe it could be important, I had to switch the default order of the monitors (DisplayPort-1 is on the left and DisplayPort-0 on the right). And again randomly, I sometimes have to redo that operation (and sometimes one of the monitor is disabled in the system settings).

I don't know if I could be more helpful but this bug drives me crazy :'(

This should be fixed in Plasma 5.25 with a big refactor of how screens are handled. Unfortunately the change was to large to be backportable to 5.24.something.

*** Bug 450446 has been marked as a duplicate of this bug. ***

Good news: we backported it to 5.24.3. :)

The screen setup is now maintained OK, but the wallpaper (different on primary screen) and the plasma widgets on are lost after rebooting.

It's probably a slightly different issue, unless you can point to unnecessary renumbering in the [ScreenConnectors] group of your ~/.config/plasmashellrc file as specifically still happening. If not, can you file a new bug report?

The screen connectors are fine, so it really seems to be a different (new) problem.

(In reply to Syiad from comment #21)
> The screen connectors are fine, so it really seems to be a different (new)
> problem.

You might be having the same error I am getting. The symptoms are already described in another (quite old) bugreport:
https://bugs.kde.org/show_bug.cgi?id=353975
Maybe the panel disappearing and the background disappearing are two separate bugs that get triggered at the same time.

It looks as if the problem happens much less often when checking the box "Save display's properties: For any display arrangement" in SystemSettings->Hardware->"Display and Monitor"->"Display Configuration"

Hello.

It just happened to me. 

I have a laptop with 2 other screens.
Now I took laptop and connected it to other 2 screens and boom, no panel at all.

> cat ~/.config/plasmashellrc
[Open-with settings]
CompletionMode=1
History=firef

[PlasmaTransientsConfig]
PreloadWeight=0

[PlasmaViews][Panel 29]
alignment=1
panelOpacity=0
panelVisibility=0

[PlasmaViews][Panel 29][Defaults]
maxLength=1920
minLength=1920
thickness=44

[PlasmaViews][Panel 29][Horizontal1050]
thickness=44

[PlasmaViews][Panel 29][Horizontal1366]
thickness=44

[PlasmaViews][Panel 29][Horizontal1920]
maxLength=1920
minLength=1920
thickness=44

[PlasmaViews][Panel 4][Defaults]
thickness=44

[PlasmaViews][Panel 4][Horizontal1366]
thickness=44

[ScreenConnectors]
0=DP-1-3
1=DP-1-1
2=HDMI-1
3=eDP-1

[Updates]
performed=/usr/share/plasma/shells/org.kde.plasma.desktop/contents/updates/systemloadviewer_systemmonitor.js,/usr/share/plasma/shells/org.kde.plasma.desktop/contents/updates/maintain_existing_desktop_icon_sizes.js,/usr/share/plasma/shells/org.kde.plasma.desktop/contents/updates/klipper_clear_config.js,/usr/share/plasma/shells/org.kde.plasma.desktop/contents/updates/no_middle_click_paste_on_panels.js,/usr/share/plasma/shells/org.kde.plasma.desktop/contents/updates/digitalclock_rename_timezonedisplay_key.js,/usr/share/plasma/shells/org.kde.plasma.desktop/contents/updates/containmentactions_middlebutton.js,/usr/share/plasma/shells/org.kde.plasma.desktop/contents/updates/digitalclock_migrate_font_settings.js,/usr/share/plasma/shells/org.kde.plasma.desktop/contents/updates/keyboardlayout_migrateiconsetting.js,/usr/share/plasma/shells/org.kde.plasma.desktop/contents/updates/move_desktop_layout_config.js,/usr/share/plasma/shells/org.kde.plasma.desktop/contents/updates/unlock_widgets.js,/usr/share/plasma/shells/org.kde.plasma.desktop/contents/updates/keyboardlayout_remove_shortcut.js

Latest openSUSE Tumbleweed.

Kernel: 6.1.8-1-default
plasma-framework-5.102.0
plasma5-workspace-5.26.5
libQt5Core5-5.15.8

Workaround from comment 13 worked for me.

Then I did:
- kquitapp5 plasmashell
- Edit ~/.config/plasmashellrc and remove all entries under [ScreenConnectors]
- kstart5 plasmashell && kquitapp5 plasmashell

and panel appeared.

And in ~/.config/plasmashellrc I found:
[ScreenConnectors]
0=eDP-1
1=DP-1-1
2=DP-1-3

Thing is, that selecting some screen as primary will make panel appear elsewhere.

I had to set 0=eDP-1 as primary to have it appear on 1=DP-1-1.

architecture-wise this changed completely in 5.27 and screen connectors are not used anywhere anymore

(In reply to Nate Graham from comment #27)
> > I'm getting the exact same problem: KDE will randomly loose the displays and panels configuration.
> Unfortunately it turns out that there are many potential root causes when
> this happens, and the root cause of the specific issue you're seeing may not
> be the same as the one for this bug report Can you submit a new bug report
> for your issue? If it does end up being the same thing, we can mark it as a
> duplicate of this bug report and then re-open it. But if it's something
> else, then it will be good to have a separate bug report for it.
> 
> Thanks!

Bug #470429 submitted