I don't know why, but I've needed to remap my super key. It got erased somehow.
I logged the keypress as 248 using xev, then used xmodmap -e "keycode 248 = Super_L" just to make sure the right key was mapped, and then I set mod4 using xmodmap -e "add mod4 = Super_L".
That set the left super key as super. But when I tried to press and hold the super key, the system doesn't recognize the hold. Too confirm this, I opened showkey, which confirms that keycode 240 release immediately follows keycode 240 press with no delay.
How do I set the system to recognize a hold of the key, rather than an immediate press-release? I need this functionality in order to use super shortcuts, obviously.