X11 invisible after upgrade to 24.05

After booting into a machine upgraded to 24.05, the laptop screen and the attached external monitor are blank, where a LightDM display manager was visible in the past.

Switching to a virtual console and logging in, confirms that the machine is booted and usable to some extent.

When switching between virtual consoles, sometimes the LightDM screen becomes visible for a fraction of a second.

I tried enabling SDDM, and the results are essentially identical: the display manager has changed but is only visible for a fraction of a second when switching virtual consoles.

Replacing SDDM with GDM seems to shift the problem slightly down the road: the GDM screen appears after boot and I can use it to log in, and then we get the same nonsense; the screen goes blank and the logged-in X session is only visible for a fraction of a second, sometimes, when switching between virtual consoles.

Have you any advice on how to debug and fix this?

My usual X11 session is fully keyboard driven, so I can get quite a bit done when being able to see the screen for only a fraction of a second every now and then. Essentially, things seem to be working fine on the displays that disappear from view as soon as they appear.

I have some xrandr screen configuration scripts which work on my pre-upgrade system, but, on the post-upgrade system, give warnings of the type

warning: DP-0 not found

What logs might contain some relevant information about the screen blanking ?

Edit: xrandr -q on the new system gives

Screen 0: minimum 320 x 200, current 2560 x 1440, maximum 4096 x 4096
None-1 connected primary 2560x1440+0+0 (normal left inverted right x axis y axis) 0mm x 0mm
   2560x1440     60.00*+

while on the old one it is

Screen 0: minimum 8 x 8, current 6400 x 2160, maximum 32767 x 32767
DP-0 disconnected primary (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)
HDMI-0 connected 3840x2160+2560+0 (normal left inverted right x axis y axis) 700mm x 390mm
   3840x2160     60.00*+  59.94    50.00    30.00    29.97    25.00    23.98  
   2560x1440     59.95  
   1920x1080     60.00    59.94    50.00    29.97    23.98  
   1680x1050     59.95  
   1600x900      60.00  
   1440x900      59.89  
   1280x1024     75.02    60.02  
   1280x800      59.81  
   1280x720      60.00    59.94    50.00  
   1152x864      75.00  
   1024x768      75.03    70.07    60.00  
   800x600       75.00    72.19    60.32    56.25  
   720x576       50.00  
   720x480       59.94  
   640x480       75.00    72.81    59.94  
DP-4 connected 2560x1440+0+0 (normal left inverted right x axis y axis) 344mm x 194mm
   2560x1440     40.00*+ 165.00  

This laptop has both an Intel and an Nvidia GPU.

Hybrid GPU mode was disabled in BIOS, because of problems with starting the X server in an earlier version of NixOS. Re-enabling hybrid mode gets around the problem. Specifically, the None-1 device disappears and is replaced with more sensible ones; xrandr -q now gives

Screen 0: minimum 320 x 200, current 6400 x 2160, maximum 16384 x 16384
eDP-1 connected primary 2560x1440+0+0 (normal left inverted right x axis y axis) 344mm x 194mm
   2560x1440    165.00*+  40.00 +
   1920x1440     85.00    75.00    60.00  
   1856x1392     75.00    60.01  
   1792x1344     75.00    60.01  
   2048x1152     59.99    59.98    59.90    59.91  
   1920x1200     59.88    59.95  
   1920x1080     60.01    59.97    59.96    59.93  
   1600x1200     85.00    75.00    70.00    65.00    60.00  
   1680x1050     59.95    59.88  
   1400x1050     74.76    59.98  
   1600x900      59.99    59.94    59.95    59.82  
   1280x1024     85.02    75.02    60.02  
   1400x900      59.96    59.88  
   1280x960      85.00    60.00  
   1440x810      60.00    59.97  
   1368x768      59.88    59.85  
   1280x800      59.99    59.97    59.81    59.91  
   1152x864      75.00  
   1280x720      60.00    59.99    59.86    59.74  
   1024x768      85.00    75.05    60.04    85.00    75.03    70.07    60.00  
   1024x768i     86.96  
   960x720       85.00    75.00    60.00  
   928x696       75.00    60.05  
   896x672       75.05    60.01  
   1024x576      59.95    59.96    59.90    59.82  
   960x600       59.93    60.00  
   832x624       74.55  
   960x540       59.96    59.99    59.63    59.82  
   800x600       85.00    75.00    70.00    65.00    60.00    85.14    72.19    75.00    60.32    56.25  
   840x525       60.01    59.88  
   864x486       59.92    59.57  
   700x525       74.76    59.98  
   800x450       59.95    59.82  
   640x512       85.02    75.02    60.02  
   700x450       59.96    59.88  
   640x480       85.09    60.00    85.01    72.81    75.00    59.94  
   720x405       59.51    58.99  
   720x400       85.04  
   684x384       59.88    59.85  
   640x400       59.88    59.98    85.08  
   576x432       75.00  
   640x360       59.86    59.83    59.84    59.32  
   640x350       85.08  
   512x384       85.00    75.03    70.07    60.00  
   512x384i      87.06  
   512x288       60.00    59.92  
   416x312       74.66  
   480x270       59.63    59.82  
   400x300       85.27    72.19    75.12    60.32    56.34  
   432x243       59.92    59.57  
   320x240       85.18    72.81    75.00    60.05  
   360x202       59.51    59.13  
   360x200       85.04  
   320x200       85.27  
   320x180       59.84    59.32  
   320x175       85.27  
DP-1 disconnected (normal left inverted right x axis y axis)
DP-1-0 disconnected (normal left inverted right x axis y axis)
DP-1-1 disconnected (normal left inverted right x axis y axis)
DP-1-2 disconnected (normal left inverted right x axis y axis)
DP-1-3 disconnected (normal left inverted right x axis y axis)
HDMI-1-0 connected 3840x2160+2560+0 (normal left inverted right x axis y axis) 700mm x 390mm
   3840x2160     60.00*+  59.94    50.00    30.00    29.97    25.00    23.98  
   2560x1440     59.95  
   1920x1080     60.00    59.94    50.00    29.97    23.98  
   1680x1050     59.95  
   1600x900      60.00  
   1440x900      59.89  
   1280x1024     75.02    60.02  
   1280x800      59.81  
   1280x720      60.00    59.94    50.00  
   1152x864      75.00  
   1024x768      75.03    70.07    60.00  
   800x600       75.00    72.19    60.32    56.25  
   720x576       50.00  
   720x480       59.94  
   640x480       75.00    72.81    59.94  
DP-1-4 disconnected (normal left inverted right x axis y axis)
  1680x1050 (0x5d) 146.250MHz -HSync +VSync
        h: width  1680 start 1784 end 1960 total 2240 skew    0 clock  65.29KHz
        v: height 1050 start 1053 end 1059 total 1089           clock  59.95Hz
  1280x1024 (0x66) 135.000MHz +HSync +VSync
        h: width  1280 start 1296 end 1440 total 1688 skew    0 clock  79.98KHz
        v: height 1024 start 1025 end 1028 total 1066           clock  75.02Hz
  1280x1024 (0x67) 108.000MHz +HSync +VSync
        h: width  1280 start 1328 end 1440 total 1688 skew    0 clock  63.98KHz
        v: height 1024 start 1025 end 1028 total 1066           clock  60.02Hz
  1280x800 (0x72) 83.500MHz -HSync +VSync
        h: width  1280 start 1352 end 1480 total 1680 skew    0 clock  49.70KHz
        v: height  800 start  803 end  809 total  831           clock  59.81Hz
  1152x864 (0x74) 108.000MHz +HSync +VSync
        h: width  1152 start 1216 end 1344 total 1600 skew    0 clock  67.50KHz
        v: height  864 start  865 end  868 total  900           clock  75.00Hz
  1024x768 (0x7d) 78.750MHz +HSync +VSync
        h: width  1024 start 1040 end 1136 total 1312 skew    0 clock  60.02KHz
        v: height  768 start  769 end  772 total  800           clock  75.03Hz
  1024x768 (0x7e) 75.000MHz -HSync -VSync
        h: width  1024 start 1048 end 1184 total 1328 skew    0 clock  56.48KHz
        v: height  768 start  771 end  777 total  806           clock  70.07Hz
  1024x768 (0x7f) 65.000MHz -HSync -VSync
        h: width  1024 start 1048 end 1184 total 1344 skew    0 clock  48.36KHz
        v: height  768 start  771 end  777 total  806           clock  60.00Hz
  800x600 (0x9a) 49.500MHz +HSync +VSync
        h: width   800 start  816 end  896 total 1056 skew    0 clock  46.88KHz
        v: height  600 start  601 end  604 total  625           clock  75.00Hz
  800x600 (0x99) 50.000MHz +HSync +VSync
        h: width   800 start  856 end  976 total 1040 skew    0 clock  48.08KHz
        v: height  600 start  637 end  643 total  666           clock  72.19Hz
  800x600 (0x9b) 40.000MHz +HSync +VSync
        h: width   800 start  840 end  968 total 1056 skew    0 clock  37.88KHz
        v: height  600 start  601 end  605 total  628           clock  60.32Hz
  800x600 (0x9c) 36.000MHz +HSync +VSync
        h: width   800 start  824 end  896 total 1024 skew    0 clock  35.16KHz
        v: height  600 start  601 end  603 total  625           clock  56.25Hz
  640x480 (0xae) 31.500MHz -HSync -VSync
        h: width   640 start  656 end  720 total  840 skew    0 clock  37.50KHz
        v: height  480 start  481 end  484 total  500           clock  75.00Hz
  640x480 (0xaf) 25.175MHz -HSync -VSync
        h: width   640 start  656 end  752 total  800 skew    0 clock  31.47KHz
        v: height  480 start  490 end  492 total  525           clock  59.94Hz

Can’t say that I understand what happened, but at least I can get on with my life.

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