Thank you Michael for your response!

Kernel

according to my irc log on #intel-gfx started to happen with upgrade to
4.2.0-1-amd64, and according to old copy of the journal it was

Oct 25 11:22:54 hopa kernel: Linux version 3.17-1-amd64
(debian-kernel@lists.debian.org) (gcc version 4.8.3 (Debian 4.8.3-13) ) #1 SMP
Debian 3.17-1~exp1 (2014-10-14)

before that.  bisection I guess will be the measure of last resort -- this
laptop is the main workhorse and halt is not 100% reproducible

patch: applied and rebuilding now.  Will report as soon as halts again (will do
some forceful playful interaction with external displays tomorrow) or if can't
trigger the halt.  Thanks!