Thanks for the bug report.
Please run:
lspci -kv > lspci.txt
journalctl -b0 > journal.txt
xrandr --verbose > xrandrv.txt
apport-collect 1913742
both before and after the problem occurs. And attach the resulting text
files here.
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** Tags added: focal multimonitor xrandr-scaling
** Changed in: mutter (Ubuntu)
Status: New => Incomplete
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https://bugs.launchpad.net/bugs/1913742
Title:
Fractional scaling: bottom half of screen black on external monitor
disconnect
Status in mutter package in Ubuntu:
Incomplete
Bug description:
I'm using Ubuntu 20.04.1, on a Lenovo Thinkpad T14s. I've got
fractional scaling enabled (125%) with Gnome.
I regularly plug into an external monitor with an HDMI connection, and
run both screens (the external monitor and the built-in laptop one)
side-by-side. With that setup, I don't use fractional scaling, because
my main external monitor is easier to read.
When I unplug the HDMI cord from my external monitor to go back to
working on the laptop, sometimes the bottom half of the laptop screen
goes black. The mouse cursor is still visible if I move it to that
region, but nothing is visible in the entire lower portion of my
screen.
I've been running 20.04 on this laptop since last August, and it
didn't used to be a problem. The problem first occurred perhaps a
month or two ago, but was rare and sporadic. It has been happening
more and more frequently, and is now at the point that most of the
time this problem occurs.
Once I have unplugged the cable and I'm stuck with a black bottom half
of the screen, I can resolve it by disabling fractional scaling, but
of course, that disables fractional scaling. At that point, any
attempts to reenable fractional scaling either bring the black region
back again, or start causing all kinds of other instabilities.
Restarting Gnome at this point contributes to the mess. The only
stable workaround I've found (apart from rebooting) is to plug the
laptop back into the external monitor again, switch the configuration
to single display mode, and then unplug. This works but is of course
cumbersome.
Is there more information I can supply that would be useful?
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