Since that bug seems related to the X server somehow, I wonder if your monitor is showing black or actually off/standby (as in backlight off)?
If it’s the backlight, maybe it’s related to DPMS (monitor power management), and you can jolt it back to life with something like
<span style="color:#323232;">xset dpms force on
</span>
after waking up. Or maybe disable DPMS completely and see if that changes anything.
It would also be interesting to know if this problem also happens outside of XFCE. If you just use (say) openbox (which I don’t think does any power management or DPMS stuff by itself), does that work?