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I have two external monitors on my kubuntu laptop, one at work 1920x1200, and one at home 1920x1080.
Half the time when I connect to either monitor, the screen gets tiled in weird ways and become unusable and forces me to reboot.

See this image for instance:

This happens both with the screen greeter (sddm as in the image above) but also if I am logged in before plugging the monitor(s).
The mouse still moves and changes cursor according to where things really are, but what I see is all wrong and immutable. The system still works, I can log remotely or via Ctrl-Alt-Console.

# lshw -c video
*-display                 
   description: VGA compatible controller
   product: TigerLake-LP GT2 [Iris Xe Graphics]
   vendor: Intel Corporation
   physical id: 2
   bus info: pci@0000:00:02.0
   logical name: /dev/fb0
   version: 01
   width: 64 bits
   clock: 33MHz
   capabilities: pciexpress msi pm vga_controller bus_master cap_list rom fb
   configuration: depth=32 driver=i915 latency=0 mode=1920x1080 resolution=1920,1080 visual=truecolor xres=1920 yres=1080
   resources: iomemory:600-5ff iomemory:400-3ff irq:160 memory:6052000000-6052ffffff memory:4000000000-400fffffff ioport:3000(size=64) memory:c0000-dffff memory:4010000000-4016ffffff memory:4020000000-40ffffffff

$ lsmod
video 61440  3 dell_wmi,dell_laptop,i915
...

When this happens I don't see anything particular in Xorg.log. I also tried killall plasmashell; kstart5 plasmashell but no avail.

How can I diagnose this further or fix it ?!?


Edit: adding xrandr output. (Note, I haven't had the crash again yet; I'll add it when it happens. Maybe @MC68020 was right...)

$ xrandr -q    # Only laptop, no external monitor
Screen 0: minimum 8 x 8, current 1920 x 1080, maximum 32767 x 32767
eDP1 connected primary 1920x1080+0+0 (normal left inverted right x axis y axis) 310mm x 170mm
1920x1080     60.00*+  59.93  
1680x1050     59.88  
1400x1050     59.98  
1600x900      60.00    59.95    59.82  
1280x1024     60.02  
1400x900      59.96    59.88  
1280x960      60.00  
1368x768      60.00    59.88    59.85  
1280x800      59.81    59.91  
1280x720      59.86    60.00    59.74  
1024x768      60.00  
1024x576      60.00    59.90    59.82  
960x540       60.00    59.63    59.82  
800x600       60.32    56.25  
864x486       60.00    59.92    59.57  
640x480       59.94  
720x405       59.51    60.00    58.99  
640x360       59.84    59.32    60.00  
DP1 disconnected (normal left inverted right x axis y axis)
DP2 disconnected (normal left inverted right x axis y axis)
HDMI1 disconnected (normal left inverted right x axis y axis)
HDMI2 disconnected (normal left inverted right x axis y axis)
HDMI3 disconnected (normal left inverted right x axis y axis)
VIRTUAL1 disconnected (normal left inverted right x axis y axis)


$ xrandr -q    # With working home monitor
Screen 0: minimum 8 x 8, current 1920 x 1080, maximum 16384 x 16384
DP-0 disconnected (normal left inverted right x axis y axis)
DP-1 disconnected (normal left inverted right x axis y axis)
DP-2 disconnected (normal left inverted right x axis y axis)
DP-3 disconnected (normal left inverted right x axis y axis)
DP-4 disconnected (normal left inverted right x axis y axis)
DP-5 disconnected (normal left inverted right x axis y axis)
LVDS-1-1 connected primary 1920x1080+0+0 (normal left inverted right x axis y axis) 382mm x 215mm
1920x1080     60.00*+  60.00  
1680x1050     60.00  
1400x1050     60.00  
1600x900      60.00  
1280x1024     60.00  
1400x900      60.00  
1280x960      60.00  
1440x810      60.00  
1368x768      60.00  
1280x800      60.00  
1280x720      60.00  
1024x768      60.00  
960x720       60.00  
928x696       60.00  
896x672       60.00  
1024x576      60.00  
960x600       60.00  
960x540       60.00  
800x600       60.00  
840x525       60.00  
864x486       60.00  
700x525       60.00  
800x450       60.00  
640x512       60.00  
700x450       60.00  
640x480       60.00  
720x405       60.00  
684x384       60.00  
640x360       60.00  
512x384       60.00  
512x288       60.00  
480x270       60.00  
400x300       60.00  
432x243       60.00  
320x240       60.00  
360x202       60.00  
320x180       60.00  
VGA-1-1 disconnected (normal left inverted right x axis y axis)

$ xrandr -q    # With working work monitor
Screen 0: minimum 8 x 8, current 3840 x 1200, maximum 32767 x 32767
eDP1 connected primary 1920x1080+1920+120 (normal left inverted right x axis y axis) 310mm x 170mm
1920x1080     60.00*+  59.93  
1680x1050     59.88  
1400x1050     59.98  
1600x900      60.00    59.95    59.82  
1280x1024     60.02  
1400x900      59.96    59.88  
1280x960      60.00  
1368x768      60.00    59.88    59.85  
1280x800      59.81    59.91  
1280x720      59.86    60.00    59.74  
1024x768      60.00  
1024x576      60.00    59.90    59.82  
960x540       60.00    59.63    59.82  
800x600       60.32    56.25  
864x486       60.00    59.92    59.57  
640x480       59.94  
720x405       59.51    60.00    58.99  
640x360       59.84    59.32    60.00  
DP1 disconnected (normal left inverted right x axis y axis)
DP2 disconnected (normal left inverted right x axis y axis)
DP2-1 disconnected (normal left inverted right x axis y axis)
DP2-2 disconnected (normal left inverted right x axis y axis)
DP2-3 connected 1920x1200+0+0 (normal left inverted right x axis y axis) 520mm x 320mm
1920x1200     59.95*+
1920x1080     60.00    50.00    59.94  
1600x1200     60.00  
1680x1050     59.88  
1600x900      60.00  
1280x1024     60.02  
1440x900      59.90  
1280x800      59.91  
1280x720      60.00    50.00    59.94  
1024x768      60.00  
800x600       60.32  
720x576       50.00  
720x480       60.00    59.94  
640x480       60.00    59.94  
HDMI1 disconnected (normal left inverted right x axis y axis)
HDMI2 disconnected (normal left inverted right x axis y axis)
HDMI3 disconnected (normal left inverted right x axis y axis)
VIRTUAL1 disconnected (normal left inverted right x axis y axis)

$ xrandr -q    # LAST EDIT: With partially duplicated home monitor. 
    # By this I mean instead of having an 'extend to left'
    # I get an unusable copy of my main screen. But it's recoverable
Screen 0: minimum 8 x 8, current 3840 x 1080, maximum 32767 x 32767
eDP1 connected primary 1920x1080+1920+0 (normal left inverted right x axis y axis) 310mm x 170mm
1920x1080     60.00*+  59.93  
1680x1050     59.88  
1400x1050     59.98  
1600x900      60.00    59.95    59.82  
1280x1024     60.02  
1400x900      59.96    59.88  
1280x960      60.00  
1368x768      60.00    59.88    59.85  
1280x800      59.81    59.91  
1280x720      59.86    60.00    59.74  
1024x768      60.00  
1024x576      60.00    59.90    59.82  
960x540       60.00    59.63    59.82  
800x600       60.32    56.25  
864x486       60.00    59.92    59.57  
640x480       59.94  
720x405       59.51    60.00    58.99  
640x360       59.84    59.32    60.00  
DP1 connected 1920x1080+0+0 (normal left inverted right x axis y axis) 530mm x 300mm
1920x1080     60.00*+
1600x900      60.00  
1280x1024     75.02    60.02  
1152x864      75.00  
1024x768      75.03    60.00  
800x600       75.00    60.32  
640x480       75.00    59.94  
720x400       70.08  
DP2 disconnected (normal left inverted right x axis y axis)
DP2-1 disconnected (normal left inverted right x axis y axis)
DP2-2 disconnected (normal left inverted right x axis y axis)
DP2-3 disconnected (normal left inverted right x axis y axis)
HDMI1 disconnected (normal left inverted right x axis y axis)
HDMI2 disconnected (normal left inverted right x axis y axis)
HDMI3 disconnected (normal left inverted right x axis y axis)
VIRTUAL1 disconnected (normal left inverted right x axis y axis)
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  • 1
    What happens after disabling compositing entirely. (system settings>Display&Monitor>Compositor) ? + what kde-plasma version installed ?
    – MC68020
    Commented Jan 13, 2023 at 14:47
  • OK, will try with compositing off. kinfocenter says 5.25.5
    – dargaud
    Commented Jan 13, 2023 at 14:54
  • And while you are at it, I suspect you are using the X-11 backend (kwin-x11), can you also retry with wayland backend ?
    – MC68020
    Commented Jan 13, 2023 at 15:16
  • 1
    And also please provide the output of xrandr -q before plugging the external monitor and of another xrandr -q after the external monitor being plugged. When everything appears correct. Then another xrandr -q when the problem occurs.
    – MC68020
    Commented Jan 14, 2023 at 9:34
  • It could be a faulty cable; can you try with another?
    – mattia.b89
    Commented Jan 15, 2023 at 8:58

1 Answer 1

1
+50

KDE multimonitor woes

Yes indeed, leading KDE devs wrote even worse :The scourge of usability.
According to Marco Martin's blog post, things should move for the better with 5.27 thanks to significant refactoring.


Most of the problems arise from the use of connector IDs to identify screens and map Plasma desktops and containments to screens.
This works poorly, because connector IDs can and do change under various circumstances. As a result, things often become a scrambled mess, with the behavior either being random, or consistently wrong.

Hence the requests for miscellaneous xrandr -q reports as part of my OP's comments.


In this context, the compositor will of course have its role in making things even worse. Hence my first suggestion to disable compositing entirely. (via the system settings>Display&Monitor>Compositor menu.

I understand from your edit that this might have helped.

But there is in fact a possibility that only the sync on vblank corrupts the compositor computations.

Therefore, if needing the compositor or willing to dig in even more, one can re-enable the compositor (via the system-settings menu) and disable the sync on vblank via the KWIN_X11_NO_SYNC_TO_VBLANK=1 environment variable setting.
Of course there is no free lunch when working-around : Do expect some screen-tearing depending on the applications you use.

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