remember the post that was made a few months ago about an infinite canvas/scrollable WM? Here we have the stable release of a (onedirectional) scrollable one inspired by gnome's PaperWM.
Isn't it the opposite then? Since your windows will have vertical scrolls, it makes sense to tile them horizontally in order to maximize vertical space for each window, imo.
That's something different. This compositor's concept is that you have line of windows that you scroll through, as you can see on the screenshots. You always see part of the line, and the part you see usually contains multiple windows. If the line is vertical as you suggests, you wouldn't usually be able to fit multiple windows on the monitor, because normal monitor is horizontal and apps are much better resizable horizontally. If you want to view two webpages at once on horizontal monitor, do you tile them vertically or horizontally?
But this is what I mean. It can still show the same amount of screen space as scrolling horizontally so there's no difference between the two options there, but it feels more natural to go up/down compared to left/right to access different content/windows.
That's just an image gallery. Applications can feel much different. There's a reason left-right tiling/snapping is much more popular than up-downs. You'd have to scroll down considerably more to grasp the content.
The only difference I can see is that you might have for example four windows 1, 2, 3, 4, all taking half of the screen. On a compositor like Niri, you can scroll so that you can see windows 1 and 2, or 2 and 3, or 3 and 4. On vertically scrolling one, you can see 1 and 2 or 3 and 4 if I understand it correctly.
This is much more noticeable if you work with many smaller windows, just like on the screenshots from the article and repo's readme.
I usually use only one or two windows per virtual desktop, so what you suggest would be more practical for me. But I use only notebook, and I can imagine using Niri on some hi-res ultrawide monitor.
Usually people like to maximize the height of windows, especially to have 2 windows side by side, so it just conceptually makes a lot of sense to have every window have the maximum height and just add windows horizontally so they are actually visible like in normal tiling window managers. Maximizing the width of windows doesn't really make that much sense honestly, because most horizontal space is wasted because theres so much horizontal space compared to vertical space.
The exact same thing would happen if you tried to scroll horizontally though? It's a 2d axis on a wheel, depending on where your focus is you're going to go the same direction.