I also don't understand tipping at food carts. Customers stand in a line to pay and get their food, and are also expected to tip (either via Square or through tip jars).

Edit: the expectation is generally implicit, and while standing in line, I can't help but notice that most customers hit the minimum tip percentage on Square, which is usually 15%

Unless you get seated and have the food delivered by the server to your table, there is no expectation of tip - no matter what Square says.

I never hit that minimum thing. I'll give $1 every once in a while because they're friendly and the town is so expensive, but the food prices already definitely reflect that, and there is no service, so /shrug

IIRC with a few extra clicks on Square you can find a menu that will give you a "no tip" option. It's definitely hidden on purpose.

ive never seen a square display without a no tip option, and I'm pretty sure the restaurant is who decides what options are there

You're right that the restaurant probably decides. The option has always been there when I looked. When it doesn't, I'll refuse to pay.

For what it's worth, I sometimes hit the minimum tip percentage solely because a colleague at work is with me. The "impulse" to hit that button is greater if that colleague is my boss. But, even when I'm surrounded by strangers, I still feel this need to hit the tip button.

Ultimately, I think right now the social expectation of tipping in this situation is in limbo- whether you do or don't, it's ok.

Were you actually requested to tip? If so, it seems strange. When I've gone to many kaiten-zushi places it is usually stated that you shouldn't tip at all (in Japan you don't). There used to be one like that downtown Berkeley off Durant that closed a few years back.

The donut shop I buy from put a new card reader system in place (probably to support chip cards). The UI requires you to enter something for a tip before you can proceed. It irritates me so much that I have stopped going as frequently. It's as if my waiter at a restaurant would hover over my table watching what I put on the tip line of the dinner bill. Infuriating dark pattern.

There was more than no service. If they work on tips then you stiffed them even thought they didn't do much.