I discovered that upgrading to kernel linux-image-4.5.0-2-amd64 fixes
this bug completely. To be clear I was on 3.14-rt beforehand and didn't
have the correct package installed for the kernel to auto-update to
newer versions.
However for the benefit of others it's probably worth noting that with
the 4.5.0-2 kernel I get another bug, which causes these errors when
switching consoles with Ctrl-Alt-F[1-9]:
[drm:intel_set_cpu_fifo_underrun_reporting [i915]] *ERROR* uncleared
fifo underrun on pipe A
[drm:ironlake_irq_handler [i915]] *ERROR* CPU pipe A FIFO underrun
[drm:intel_set_pch_fifo_underrun_reporting [i915]] *ERROR* uncleared pch
fifo underrun on pch transcoder A
[drm:cpt_irq_handler [i915]] *ERROR* PCH transcoder A FIFO underrun
Sometimes this can cause a crash. So I've compiled the mainline kernel
4.7.0-rc1 (not tested any lower versions yet). Finally everything seems
to be working OK!
