Hi all,
I'm facing a rather peculiar issue and I need some hint on how to debug it.
Basically, after logging in the notification "Desktop effects have been suspended by another application" pops up, but not always, it is kind of random, happens say 60/70% of times (rough extimate). a the same time, the system setting module for compositor apepars.

If, after the "effect disabled" popup appears, I hit alt/shift/f12 I get immediately the same popup again.

However, if the popup does not appears at logon, the session goes on without problems with compositor enabled.
I'm using the laptop in three different arrangements: standalone, one external monitor+intenal LCD and two external monitors and lcd turned off, frequently switching across them, but I spotted no pattern that correlates the monitor setup to the compositing issue.
By the same token, I have yet to find any possible culprit among the applications that are started at logon, any change seems to have no effects.
I'm really puzzled about how to detect the issue, definitely not a big one but somewhat annoying.
Is there any way to detect which application is causing kwin to turn off compositing? I tried to look at logs, dbus, etc.. but I fail to find any specific info so any help would be appreciated.
System:
Gentoo
Linux Kernel 4.16.6 Vanilla
Gcc 7.3.0
mesa 18.1.0_rc2 (the issue is here since 17.3.8 or before, IIRC)
kwin 5.12.4
qt 5.9.5
plasma 5.12.4
the laptop is using nvidia drivers with prime enabled.

Of course I can provide any additional info that can be useful.

Most probably an app requested the system to disable effects so it can be faster to do something, this happen mostly with games. You can disable that behaviour in:

Settings > Display > Compositor > Uncheck "Allow apps to disable compositing".