This reminds me of the chat situation where a coworker will start with "Hi!" typing indicator.
In either case, it's like "yes, you have my attention. I cannot focus on anything at all until you hit enter, and I also can't reply because I have nothing to reply to".
I also can't complain like "hey if you're going to interrupt me can you just say the whole message so I can actually reply instead of being held in absolute suspense for many seconds of my life?" because that would be, frankly, even worse than what they're doing, which was done out of a kind of ignorance of etiquette.
I'll respond with "hi", except I'll wait a few seconds to try and break their train of thought. Sometimes it works and the typing indicator goes away for a bit. I think I get less of these lone "hi" messages than I used to, so it may even be having an effect on them.
It really only works if there's a typing indicator and they've already started the next message, otherwise you're just giving them what they want and reinforcing the bad habit.
What I do in this instance is mute that person for an hour or so and leave to do what I was doing. It means a less quick response to the person but keeps me sane a lot of the time.
In either case, it's like "yes, you have my attention. I cannot focus on anything at all until you hit enter, and I also can't reply because I have nothing to reply to".
I also can't complain like "hey if you're going to interrupt me can you just say the whole message so I can actually reply instead of being held in absolute suspense for many seconds of my life?" because that would be, frankly, even worse than what they're doing, which was done out of a kind of ignorance of etiquette.