I encounted 2 problems that happen only on Xorg and not on Wayland. But since Xorg is still very common among KDE Plasma users I think this problems should get a fix. While pressing Volume Up, Volume Down, Mute, Brightness Up or Brightness Down, and if there is a QT app on top, the "blue side" of the Qt app will look strange. With "blue side" I mean the part the app is selected. For example: if I press one of the above mentioned buttons while using Audacious (QT interface), some blue parts will get lighter and then darker again. There is one more problem: if I am listening to a Song from Soundcloud (for example, but it could also happen with youtube) with the played song bar in the middle of the screen, and if I then press Volume Up or one of the mentioned keys, the "notification at the centre of the screen" will likely be showing well for the few moments, but in the end it will be flickering. This can be seen better with a Dark Theme. It would be very nice if these bugs can get solved on Xorg :) Thank you for your attention!
Which version of Plasma are you using? If I understand this report correctly, the OSD steals focus.
As you can tell this is fixed on Wayland. It cannot be fixed on X. On X the way global shortcuts work is that we register a "soft key grab" which means X server will grab the key once its pressed, we register that, release the grab again, and the invoke whatever action has been configured. This unfortunately also means that the active window will briefly lose focus leading to the flickering you described. As for the OSD, I noticed that too but the blur effect is currently being rewritten which might also solve this.
I'm using KDE Plasma 5.11.5 but I've been experiencing this since my KDE Neon installation. Yes, the OSD is stealing focus. Thank you for your answers! Hope on a fix, otherwise I'll just have to switch to Wayland! Cheers!