gnome-control-center chooses modeline that cuts signal to external monitor
I'm using Ubuntu 20.04, mutter 3.36.4-0ubuntu0.20.04.2. This issue appears in both Xorg and Wayland.
The bug is that if I use g-c-c to choose 1920x1080 as the resolution for my external 2560x1440 monitor, the monitor loses signal and turns off.
xrandr --verbose shows that g-c-c has selected this modeline (note the vertical frequency of 23.98Hz):
1920x1080 (0xc6) 74.176MHz +HSync +VSync *current h: width 1920 start 2558 end 2602 total 2750 skew 0 clock 26.97KHz v: height 1080 start 1084 end 1089 total 1125 clock 23.98Hz
If I use the command "xrandr --output HDMI-1 --mode 1920x1080", the monitor switches correctly to the selected resolution and remains on. xrandr --verbose says it is using this modeline:
1920x1080 (0xbc) 148.500MHz +HSync +VSync *current h: width 1920 start 2008 end 2052 total 2200 skew 0 clock 67.50KHz v: height 1080 start 1084 end 1089 total 1125 clock 60.00Hz
Note that if I can successfully choose a resolution of 1600x1200 using g-c-c.
There is more information including full xrandr --verbose and journalctl dumps at https://bugs.launchpad.net/ubuntu/+source/gnome-control-center/+bug/1892296.