Person I know tries to sneak up on me in the cafeteria / stands behind me?

I was at the cafeteria at my college, eating some food while working on some homework. At one point, I turn around and see a guy I know simply standing behind me, leering over my shoulder and grinning.

I then loudly exclaim (embarrassingly, a lot of people noticed) “What the actual fuck do you think you’re doing?” and he is a bit taken aback. He then tells me he was just messing around and that he took a photo of me and sent it to me on Discord too. He then asks me something about my homework as well. Someone standing over my shoulder, without informing me of their presence, and that too reading what I’m doing on my computer is not fun for me.

He did this once before too, but I did tell him something like “Oh, you should’ve just said hi, don’t just’ stand there”. He probably finds it funny or something like that, but I did tell him not to do it, prior. And as today had already been a hard day for me, I think I reacted far stronger than I otherwise would have.

I know we all have our boundaries that we like to communicate but I’m curious – isn’t this a pretty obvious one someone shouldn’t break?