Public bug reported:

Two identical Lenovo E495 laptops with 20.04 installed. The problem
occurred initially on the laptop that is having package upgrades applied
regularly.

With dual monitors and the external monitor placed left or right the
display has a blocked staircase effect shown in the attached photograph,
and seems related to

https://gitlab.freedesktop.org/xorg/driver/xf86-video-amdgpu/-/issues/10

More detailed investigation suggests it only happens when the X
coordinate of the two monitors is different. The symptom looks like an
off-by-one error because it appears as if the display is divided into,
say, 10 rows and 15 columns but the first row has 16 'columns' worth of
blocks on it and so wraps to the beginning of the 2nd row, and so on.

On the laptop without package upgrades being applied this didn't happen.
So I upgraded it (314 packages) and restarted and it too sees the same
problem.

I suspected libxcomposite1 and downgraded it to 1:0.4.5-0ubuntu1 but
that didn't solve it.

I now suspect libxcb but so far haven't been able to prove it.

** Affects: libxcb (Ubuntu)
     Importance: Undecided
         Status: New

** Attachment added: "Blocked Staircase across two monitors"
   
https://bugs.launchpad.net/bugs/1873895/+attachment/5357157/+files/IMG_0023.JPG

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https://bugs.launchpad.net/bugs/1873895

Title:
  Regression: block staircase display with side-by-side monitors

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