I’m encountering this issue as well. It will occur if a desktop notification appears, regardless of whether the window is focused, minimized, or closed. Upon occurrence, I have to force kill all Mattermost processes using task manager. Process will re-occur upon restart of Mattermost if another notification appears.
I have no tried rebooting or reinstalling, and am not planning to.
Workaround is disabling desktop notifications for each server I use, as there is no global option in the app AFAIK.
System: Windows 10 Enterprise x64 build 14393