Applications do not repaint in a timely fashion, resulting in screen corruption

Overview

I am using a Dell XPS 13 7390 (i7-10710U integrated video) with modesetting 2:1.20.8-2ubuntu2.4 on Ubuntu 20.04 and kernel 5.4.0-42-generic on the same. I have reported this to Ubuntu via this Launchpad bug, but they requested that I report it here:

https://bugs.launchpad.net/ubuntu/+source/xorg-server/+bug/1898210

I experience delayed repaints in many applications, which resolve only when the application is disturbed or the application focus changes to another window. I am attaching a video of this problem:

Firefox 80.0.1 corruption.

Affected applications

• rxvt-unicode v9.22
• Firefox 80.0.1
• Emacs 26.3
• Xournal 0.4.8
• Various Electron apps
  • Slack
  • Element
  • Discord
  • Keybase
• others!

Desktop environment

My WM is Awesome v4.3. I have reproduced this bug both without a compositor running (the video attached above has no compositor running) and with compton; it does not appear to make a difference.

I have three displays attached:

• eDP-1, a 3840x2160 laptop display panel
• DP-2-8, a 1920x1080 DisplayPort monitor connected to the laptop via USB C
• DP-2-1, a 1980x1080 DisplayPort monitor connected to DP-2-8 via DisplayPort chaining

My desktop is 11520 x 2160, and the two external monitors are scaled from 3840x2160. I use the following XRandR configuration:

HIX=3840
HIY=2160
HI=${HIX}x${HIY}
xrandr --fb $(($HIX * 3))x$HIY \
       --output eDP-1 --auto --panning ${HI}+$(($HIX * 2))+0 \
       --output DP-2-8 --auto --panning ${HI}+$HIX+0 \
                       --scale-from $HI --left-of eDP-1 --scale 2x2 \
       --output DP-2-1 --auto --panning $HI+0+0 \
                       --scale-from $HI --left-of DP-2-8 --scale 2x2

Other maybe-related symptoms

With the three displays attached, GL performance on eDP-1 is very bad -- something like 1/4 the performance of either external display best case, and sometimes far worse, according to glxgears with vertical blanking synchronization disabled. Electron applications are nearly unusable on eDP-1 without --disable-gpu, as simple typing takes several seconds for keystrokes to appear.

There are glitches when configuring DP-2-8 for scaling; it doesn't "take" the first time, the display port is about 1/2 a screen too far to the left, and pans when the mouse hits the borders. Re-running the configuration for just DP-2-8 via xrandr fixes it up and it behaves normally thereafter.

Related bugs

https://bugs.launchpad.net/ubuntu/+source/xorg-server/+bug/1898210 https://bugs.launchpad.net/ubuntu/+source/emacs/+bug/1893969

Both have videos and screen shots attached (the video on the first of these bugs is attached to this bug).