There are cases in which applications are able to freeze the screen (here is an example bug report: https://bugs.winehq.org/show_bug.cgi?id=39036 ). I have seen this also if the debug version of the Flash Player shows a backtrace while an entry in the menu bar of Firefox is opened but in this case anything freezes except the mouse cursor. I think the xserver should not allow applications to be this mighty and always guarantee that the user have always control over the screen.
I just've seen that bug I wanted to report, too. I figured out when testing a Java-Application which freezed my desktop till I killed the process and waited a few seconds. I attached the source. I tested it so far on Manjaro (KDE), Deepin OS and LM 18 Cinnamon. So it seems to be releated to Xorg. However different desktops may result in different freeze-times due to diffent compositors and window-management-applications like cairo-dock. I also attached a screenshot of htop during freeze which show the processes which are affected.
Created attachment 128676 [details] Freeze-Example (Java-Source) Comments and instructions to run are in the file
Created attachment 128677 [details] htop during freeze Htop-Output opened on a tty2 (CTRL + ALT + 2) during freeze with the example Java-Code.
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