Word for the typical posture of a smartphone user

The actual physical posture, without imputing any behavior or moral characteristic to it, is "hunched". It often takes the preposition "over".

There are a bunch of kids hunched over their smartphones; I don't know if they're texting or playing a game.

While we're visiting your grandparents, I don't want to see you hunch over your phone.

Why is Jim all hunched over? I don't know - did he hurt his back?

Hunch (MW intransitive verb, definition 2a)

to assume a bent or crooked posture

Maybe it should be "slouched over their smartphones", because "leaning on their smartphones" doesn't make sense, you can't lean over your smartphone. When you are leaning on something, you are leaning on a wall or something, therefore, leaning over is more like bending over/tilting over. Slouched is more like bending, but not a lot.

(M-W)