If you can be “in a mode, but only temporarily” – like the shift key puts you in capital letter mode “only temporarily”, as opposed to how CapsLock does it “permanently” (until you press it again) – what does that prove?

AFAICS only that there is a mode.

Glue some little mechanical contraption onto the Cat's keyboard to hold down the Search key until you nudge the thingymabob again to release it, and it's just as (semi-)“permanent” as CapsLock. (AFAIK, that's exactly how CapsLock worked on mechanical typewriters.) The fact that they didn't build that into the Cat's keyboard in the first place is just an implementation detail... That seems very much like a feint to avoid admitting that there's no “quasi” about it; a mode is a mode is a mode.

Those are not equivalent; barring some highly unusual configuration, the Shift key has no effect on application state outside the scope of the keypress. In that sense it seems to me that Leap leaks.