Ok, I'm not in the target audience. Fine. I would still like to know what it is.

Well, the first thing you need to know is that, as the title of the submission clearly states, this is about touchpads. Not keyboards. So, y'know, pointing out that the down arrow isn't super relevant isn't exactly coming out of nowhere.

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.

I see.

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.

On older systems, scrolling on the trackpad would emulate mouse scroll wheel events - scrolling my one (or more) lines per increment.

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.

I finally get what this is about. That sounds terrible. Thank you.

Scrolling that exactly matches your fingers and has “realistic” momentum.

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.

I just tried it on this Windows laptop, and it worked exactly as you described, at least in firefox. I pretty much never use a touchpad though, so I don't think I ever saw that before. If I did though, I can imagine coming to rely on it pretty quickly.

From Macrumors: MacBook Pro offers an enhanced multi-touch trackpad supporting inertial scrolling. The feature, already present in similar forms on Apple's iPhone OS devices and the Magic Mouse, allows users to "flick" while scrolling as the trackpad senses the momentum of the gesture and smoothly scrolls through long documents and libraries.