Created attachment 106160 [details]
This is the crash dump mentioned in syslog, a copy of /sys/class/drm/card0/error

Got this when Alt-`ing from one terminal window to another.
The display froze for 10 or 20 seconds and then came good.
Tailing /var/log/syslog I saw
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Sep 12 08:42:18 nimbus kernel: [11808.520092] [drm:ring_stuck] *ERROR* Kicking stuck semaphore on render ring
Sep 12 08:42:18 nimbus kernel: [11808.520096] [drm] GPU crash dump saved to /sys/class/drm/card0/error
Sep 12 08:42:18 nimbus kernel: [11808.520097] [drm] GPU hangs can indicate a bug anywhere in the entire gfx stack, including userspace.
Sep 12 08:42:18 nimbus kernel: [11808.520098] [drm] Please file a _new_ bug report on bugs.freedesktop.org against DRI -> DRM/Intel
Sep 12 08:42:18 nimbus kernel: [11808.520098] [drm] drm/i915 developers can then reassign to the right component if it's not a kernel issue.
Sep 12 08:42:18 nimbus kernel: [11808.520099] [drm] The gpu crash dump is required to analyze gpu hangs, so please always attach it.
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I'm not on a bleeding-edge build but the message asked me to report, so here it is.

John Mills