With the help and suggestions of Bluesabre on #xubuntu-devel we've tracked it down to a recent change in xfwm4 where it switched from using glx to xpresent:
https://git.xfce.org/xfce/xfwm4/commit/?id=23900123ad8418149897a094d1096d6ecb984d3c which seems to be a bug in the driver as mentioned orginally: https://bugzilla.xfce.org/show_bug.cgi?id=16716 => https://gitlab.freedesktop.org/xorg/driver/xf86-video-amdgpu/-/issues/10 It is also SPECIFIC to the resolution selected on the external HDMI display. In our case the Lenovo E495 has eDP 1920x1080 and external HDMI are 168x1050 and are affected. I tried with another external HDMI that is natively 1920x1200 and couldn't reproduce UNTIL I set it to use 1680x1050 instead of its preferred resolution, at which point the blocked staircase returned. As I said above it feels like an off-by-one bug on the assumption rendering the display is split into regions (columns) and it is rendering 1920 pixels to a 1680 wide output with the pixels 1680-1919 wrapping to the next 'row'. ** Bug watch added: Xfce Bugzilla #16716 https://bugzilla.xfce.org/show_bug.cgi?id=16716 ** Bug watch added: gitlab.freedesktop.org/xorg/driver/xf86-video-amdgpu/-/issues #10 https://gitlab.freedesktop.org/xorg/driver/xf86-video-amdgpu/-/issues/10 -- You received this bug notification because you are a member of Ubuntu Bugs, which is subscribed to Ubuntu. https://bugs.launchpad.net/bugs/1873895 Title: Regression: block staircase display with side-by-side monitors To manage notifications about this bug go to: https://bugs.launchpad.net/ubuntu/+source/libxcb/+bug/1873895/+subscriptions -- ubuntu-bugs mailing list ubuntu-bugs@lists.ubuntu.com https://lists.ubuntu.com/mailman/listinfo/ubuntu-bugs
