Comment # 18 on bug 909235 from Roger Whittaker

Hmm - this is very strange.  I was seeing this behaviour consistently earlier
in the day, and now (after undocking / redocking a number of times, but
changing nothing in the configuration) I cannot reproduce it any more.

I'll test more tomorrow, but if it's not happening consistently then maybe this
is INVALID, and caused by some hardware problem with the dock(?)

I can't see any events via evtest when the monitor goes away - at least not in

/dev/input/by-path/platform-thinkpad_acpi-event

An added complication is that the dock's output to the HDMI switcher is a
display-port to HDMI cable.