When I press volume up/down keys on my multimedia keyboard, the indicator overlay sometimes steals focus from the applications, which results in unwanted behavior. For example, many games tend to jump into the menu or even minimize the whole window, because the focus has been lost (just for a split second, but that is enough). This doesn't seem to happen on a single press of the volume up/down key, but when I hold it for a while longer, so that multiple volume steps are performed (e.g. when you want to quickly raise or lower the volume), the focus is stolen. This only happens under X11, not under Wayland.

The reproducer is trivial with Cannon Brawl Demo [1] (free on Steam):

1. run X11 session
2. start Cannon Brawl Demo [1] in fullscreen mode
3. hold volume up or volume down on your multimedia keyboard for a second or two
4. the game will minimize (100% reproducible)
5. you can try the same in windowed mode, the game will display a message "window out of focus"

[1] http://store.steampowered.com/app/325760/Cannon_Brawl_Demo/

gnome-shell-3.22.3-1.fc25
mutter-3.22.4-1.fc25