those wanting to know about this bug may visit https://bugs.launchpad.net/ubuntu/+source/xfwm4/+bug/1001936
i advised in my previous post that xfwm4_4.8.3-1ubuntu1.1_i386.deb from ubuntu was supposed to have the fix, but did not solve the problem for me. i have since updated to xfwm4_4.10.0-2ubuntu1 from package manager.
the problem still comes, but it does so once in a while i.e. no longer as often as before.
as i'm doing this post, it happened again.
on a side note, i have never had the same problem in Carolina Puppy which has xfce4.10, but without gtk3 (i think).
you mentioned you use 64 bit version which is maybe why you are unaffected.
if there's any difference for the xfwm4 and gtk3 (the 2 culprits) between 32bit and 64bit mint xfce, i would not know.
the ubuntu bug tracker does not seem to discriminate, so all i can say is you are one lucky guy for not encountering this problem (did you not really?)