Can't interactively unify in some cases

Why would the following behavior occur: in some cases, dragging a structure onto a [sign] doesn’t lead to either a unification result or a unification failure; no new window opens. It just seems to silently ignore the action?..

Screen recording (10 seconds).

I’m afraid I really don’t know. It might help to specify exactly which combination of software you’re using though?

I tried both LKB FOS with the new LUI and the classic logon…