About 1 week ago, I installed MATE desktop (Gnome2 fork) because I wanted to use XMonad in gnome. So I am pretty sure this problem has something to do with MATE.
My keyboard is just a basic HP keyboard (US layout); it doesn't have any specialised media/audio keys. So I decided, that since I never use the Alt_Gr, Super_L and Menu keys (3 consecutive keys), I could get the desktop environment to use those effectively as 'media keys'. Until I switched to MATE, I had been using XFCE and this worked very nicely.
However now that I try to reconfigure this in MATE, I found that these 3 keys specifically are not detected by the keyboard-shortcuts-setup-utility-thing. I suspected maybe XFCE was blocking them or something, but resetting XFCE's shortcuts to defaults didn't fix it. I even tried manually editing the shortcuts in mateconftool-2 (gconf), without success.
I have not been able to detect these 3 keys in any way, and
dmesg shows nothing of interest. The only way I know they are working is that borders in programs jump, or do something, whenever I press one of these keys.
Are there any other tests I can run, or other suggestions?