After some period of time (typically 24h to 48h), graphical applications stop responding. Starting a new session in a virtual console everything works fine. Attempting to terminate the graphical user session with `loginctl terminate-session` results in lots of zombie processes. Processes remain after `killall -u <user> -9`. Killing `killall Xorg` does kill the X server, which restarts as expected and the display manager dialog box appears. However, the zombie user processes still remain and a login (by the same user) fails with `/usr/bin/startxfce4: X server already running on display :0`. Shutting down the system, one of the last messages appearing on the console is a list of unkillable processes. Reproducible: Always Steps to Reproduce: 1.Start xfce4 session. 2.Wait up to several days. 3. Actual Results: Apps stop responding. Expected Results: Applications continue to work as normal. This has only been seen on systems with Nvidia graphics cards (GeForce GT 1030 and GeForce GTX 660) but with both the nv driver and the Nvidia proprietary driver (multiple versions) though the nv driver appears to have additional issues which may confuse the picture. The problem does not appear running the lxde desktop.