I hope KWin is the appropriate area for this problem.

I am using Plasma 5 under Kubuntu 18.04.

Under System Settings > Window Management > Window Actions and Behavior -> Title Bar: Double-click I have set the action to "Maximize (horizontal only)".

For reasons unknown and a scenario I can't duplicate that functionality disappears. Double-clicking a title bar does nothing. Generally, it only disappears when the system is up going on 48 hours or more. I have an alias to restart Plasma shell which executes 'kquitapp5 plasmashell ; plasmashell &' but restarting plasmashell does not restore the functionality as set above. I can only restore that setting by reboot.

What process should control this aspect and what can I restart to gain it back without having to reboot the PC?

Thanks for taking the time to read this.

Why reboot? Logging out and back in again restarts the GUI, that should be enough.

You could also file a bug report to https://bugs.kde.org/enter_bug.cgi?prod ... nent=rules