While a menu in an application is open, the keyboard keys to control volume, media playback, are blocked. Easily reproducible by opening the Edit or View menus in any application, and trying to use the keyboard media keys.
I'm afraid this is an irresolvable problem on X11 due to the way X11 handles keyboard input. Fixed on Wayland though! :) We really need a CANTFIX or UNFIXABLE sub-status. INTENTIONAL is not really accurate.
*** Bug 432581 has been marked as a duplicate of this bug. ***
*** Bug 460579 has been marked as a duplicate of this bug. ***
*** Bug 470245 has been marked as a duplicate of this bug. ***