Created attachment 111427 [details] Video of the result seen from someone else on a chat client. Adjusting the volume with keyboard control will cause the currently focused window to loose and gain focus repeatedly, for each steps of volume change, for the whole time of the adjustments. In some chat clients, like telegram or steam chat, this will cause contacts to be spammed with indications that you're idling and that you are online.
That's Plasma OSD. You can disable it completely in System Settings.
That's not the OSD, that's KGlobalAccel setting a soft keyboard grab on the shortcut. When it is pressed, it grabs keys, we detect it and ungrab it. Unfortunately, for a short time the key is grabbed resulting in focus loss. Fixed in Wayland, unfixable on X.