I have a Macbook from 2008 (Intel Core2 duo) running Ubuntu 20.04. The primary display (on the laptop screen) is fine, and I'm plugging the external monitor in through a DVI-mini display adapter. The 2nd display (external) comes through, but at the default resolution (1920x1080) is unstable. It's like watching old DVDs with bad compression or connection.
When making adjustments with "Displays", the whole system locks up whenever I click apply. I've rebooted and tried this again, and the same thing has happened about 4 times now. Everytime I try to change anything in Displays with the external plugged in, the system locks up and I have to reboot. With just a the laptop monitor alone, I can make changes.
Seems like a bug with the "Displays" applet. Any suggestions would be welcome. Here is the output from xrandr:
$ xrandr
Screen 0: minimum 320 x 200, current 3200 x 1080, maximum 8192 x 8192
LVDS-1 connected primary 1280x800+1920+280 (normal left inverted right x axis y axis) 286mm x 179mm
1280x800 60.22*+ 59.99 59.97 59.81 59.91
1280x720 60.00 59.99 59.86 59.74
1024x768 60.04 60.00 59.95
960x720 60.00
928x696 60.05
896x672 60.01
1024x576 59.95 59.96 59.90 59.82
960x600 59.93 60.00
960x540 59.96 59.99 59.63 59.82
800x600 60.00 60.32 59.96 56.25
840x525 60.01 59.88
864x486 59.92 59.57
800x512 60.17
700x525 59.98
800x450 59.95 59.82
640x512 60.02
720x450 59.89
700x450 59.96 59.88
640x480 60.00 59.94 59.94
720x405 59.51 58.99
720x400 59.97
684x384 59.88 59.85
680x384 59.80 59.96
640x400 59.88 59.98 59.96
576x432 60.06
640x360 59.86 59.83 59.84 59.32
640x350 59.84
512x384 60.00
512x288 60.00 59.92
480x270 59.63 59.82
400x300 60.32 56.34
432x243 59.92 59.57
320x240 60.05
360x202 59.51 59.13
320x180 59.84 59.32
DP-1 connected 1920x1080+0+0 (normal left inverted right x axis y axis) 531mm x 298mm
1920x1080 60.00*+ 59.96 59.93
1680x1050 69.88 59.95 59.88
1600x1024 60.17
1400x1050 74.76 70.00 59.98
1600x900 59.95 60.00 59.82
1280x1024 75.02 60.02
1440x900 59.89
1400x900 59.96 59.88
1280x960 60.00
1440x810 59.97
1368x768 59.88 59.85
1360x768 59.80 59.96
1280x800 59.99 59.97 59.81 59.91
1152x864 75.00 75.00 70.00 60.00
1280x720 60.00 59.99 59.86 60.00 59.74
1024x768 75.05 60.04 75.03 70.07 60.00
960x720 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 75.00 70.00 65.00 60.00 72.19 75.00 60.32 56.25
840x525 74.96 69.88 60.01 59.88
864x486 59.92 59.57
800x512 60.17
700x525 74.76 70.06 59.98
800x450 59.95 59.82
640x512 75.02 60.02
720x450 59.89
700x450 59.96 59.88
640x480 60.00 75.00 72.81 75.00 59.94
720x405 59.51 58.99
720x400 70.08
684x384 59.88 59.85
680x384 59.80 59.96
640x400 59.88 59.98
576x432 75.00 75.00 70.00 60.06
640x360 59.86 59.83 59.84 59.32
512x384 75.03 70.07 60.00
512x288 60.00 59.92
416x312 74.66
480x270 59.63 59.82
400x300 72.19 75.12 60.32 56.34
432x243 59.92 59.57
320x240 72.81 75.00 60.05
360x202 59.51 59.13
320x180 59.84 59.32
Best solution I found so far was to install mate
$ sudo apt install ubuntu-mate-desktop(During the installation I selected lightdm instead of gdm3. I believe that's the same as the default for ubuntu, so I don't know why this made a difference.)Then log-out, and log back into a Mate session (which I recommend for lots of other reasons anyway.) I don't know how to resolve this for general GNU sessions, but for me, this totally fixed the display instability, somehow, and I prefer Mate anyway, so I'm happy.