Thanks. There seems to be a recurring issue with a monitor vanishing
from the known list of monitors:

de des. 16 10:44:51 laptop-dani-g gnome-shell[4343]: 
(../src/backends/meta-output.c:142):meta_output_get_monitor: runtime check 
failed: (priv->monitor)
de des. 16 10:44:51 laptop-dani-g gnome-shell[4343]: 
(../src/backends/meta-output.c:142):meta_output_get_monitor: runtime check 
failed: (priv->monitor)
...
de des. 16 10:44:57 laptop-dani-g gnome-shell[4343]: 
(../src/backends/meta-output.c:142):meta_output_get_monitor: runtime check 
failed: (priv->monitor)
de des. 16 10:44:57 laptop-dani-g gnome-shell[4343]: 
meta_monitor_manager_get_logical_monitor_from_number: assertion '(unsigned int) 
number < g_list_length (manager->logical_monitors)' failed
de des. 16 10:44:57 laptop-dani-g gnome-shell[4343]: 
meta_workspace_get_work_area_for_monitor: assertion 'logical_monitor != NULL' 
failed
de des. 16 10:44:57 laptop-dani-g gnome-shell[4343]: 
meta_monitor_manager_get_logical_monitor_from_number: assertion '(unsigned int) 
number < g_list_length (manager->logical_monitors)' failed
de des. 16 10:44:57 laptop-dani-g gnome-shell[4343]: 
meta_workspace_get_work_area_for_monitor: assertion 'logical_monitor != NULL' 
failed

Although some of that I would say is undeniably a bug in Mutter (the
compositor), because it should be tracking monitors with less confusing
messages, what triggers it might be lower level. Lower level like the
kernel or hardware is reporting the monitor as disconnected.

The first thing I would try is to delete the monitor config
(~/.config/monitors.xml) and log in again. Just in case the cause is
some weird configuration and resetting to defaults avoids it.

If that doesn't solve the problem then next I would try a new display
cable, because those are often the issue:
https://discourse.ubuntu.com/t/the-display-bug-we-cant-fix/57434

Avoiding HDMI completely is also advisable if you can get a USB-C to
whatever adapter cable. This is because laptops often have cheap low
quality HDMI ports and running the display signal though the USB-C port
in Alt mode avoids that.


** Changed in: mutter (Ubuntu)
       Status: Incomplete => New

** Tags added: i915

** Summary changed:

- External screen black after sleep
+ [i915] External screen black after sleep

** Also affects: linux (Ubuntu)
   Importance: Undecided
       Status: New

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https://bugs.launchpad.net/bugs/2136133

Title:
  [i915] External screen black after sleep

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