Bug#929834: Also seeing it with gdm3

[John Goerzen <jgoerzen@complete.org>]

Hi Folks,

On a Dell Latitude with Intel graphics, running gnome flashback, I also
saw this behavior after upgrading from stretch to buster.  This laptop,
however, is running gdm3, not lightdm.  I am seeing the exact same
symptoms, with VT switching generally working around the issue
post-lock.

So far, the workaround at
https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=929834#66 has resolved
this for me.  I will update this bug if I see any further issues.

John