What exactly is smooth scrolling? Like you press the down arrow, and the scroll is animated instead of instantly snapping down by 50 pixels?
It's supposed to be like "buttery smooth" scrolling on a smartphone. Whatever is on the screen should move with your fingers, with pixel accuracy and low delay, so it feels like you're dragging something around with your fingers.
When you let go, it might continue to scroll, as if you flicked the object with your fingers. This is called inertial scrolling. At the limits, it should show your attempt to scroll beyond the limits somehow. Apple and Android do this differently for patent reasons.
Of course with a touchpad the image isn't underneath your fingers like with a smartphone. So the scrolling amount doesn't have to be the same physical length as your finger movement. It's scaled, but it should feel similar to smartphone scrolling.
There's no arrow.
Privately I am using a lot of Chromebooks and it is good in my opinion, but I was never wowed by Apple implementation so maybe I am not a good person to ask.
On a Mac I might scroll through an article by just sort of pushing it the right direction, removing my hand from the trackpad and then tapping to stop it at the right place. It’s very hard to describe these things because so much is muscle memory.
But assuming that their version of smooth scrolling does, in fact, work the same as Apple's, it's not even a matter of "it smoothly animates scrolling down by one line;" it's that you can scroll by individual pixels, rather than by lines, using the touchpad. I suspect that a certain amount of work also has to go into ensuring that the scroll animation is both smooth and well-synced with the user's finger motion on the touchpad, but I've never done work that low-level, so I'll have to defer to anyone with better expertise there.
If you can move a mouse cursor by a single pixel, that would seem to be enough fidelity to scroll by a single pixel. I've never seen a touchpad that didn't meet that criteria. But then I've never used a touchpad in linux. And maybe I'm just wrong about everything. Wouldn't be the first time.
This made sense with scroll wheels, because they moved in discrete (large) clicks.
The problem isn't capturing one-pixel accuracy with the trackpad deiver, it's that at the application layer a lot of legacy mouse input APIs treat scrolling as if the user still has a 90s-era mouse with a physical scroll wheel.