> what the fuck am I supposed to read next?
What a weird comment. You read whatever you want next, ever read a newspaper? You scan it all and pick the article you are interested in, then read that. I don't understand these comments, they work perfectly well in real life (and fixed size is arbitrary, I can make a super wide or super long newspaper too, the axis size does not affect this sort of layout, see infinite scroll for example, as there is only a fixed amount of content on the screen at any given time).
> You scan it all and pick the article you are interested in
Okay. What order am I supposed to scan in so I don't lose my place and accidentally skip a block? Scanning column by column gets me cut off partial boxes that I'll have to remember to check again later, while scanning side to side forces me to keep track of each individual block I've already looked at, as opposed to a single pointer to "this is how far I've scanned". Alternatively, I can scan roughly left to right, top to bottom and just live with missing some blocks. That's not ideal either, because hopefully if you didn't think I'd like to look at all of them you wouldn't have included them on the page.
You're right that you can make a newspaper that's really inconvenient to read, but you wouldn't, because the failure case you'd end up with is CSS Grid Lanes.
This is so funny that I'm not even sure what to say. You can ask your exact questions about a newspaper but somehow 99% of people manage to read them just fine. I think it's just a you problem that you are looking for an exact algorithm on how to scan a page with multiple sizes of content; in reality, people just look over it all and keep track of what they have or haven't looked at all in their heads.
In a newspaper the answer is simple. You linearly scan the leftmost column to the bottom of the page, then the next column, then the next, and so on until you get to the end of the page. At no point do you ever need to keep track of anything other than "this is how far I've gotten" to make sure you haven't missed anything. Columnar layout make sense in newspapers because both axes are fixed in size, so all you ever do is one long linear scan with wraparound.
If one axis is fixed, and it is in the case of grid lanes (it's not a fully pannable infinite canvas like Figma after all), you just keep reading the content that's on the current screen, then you scroll. I really don't see how it's any different to, for example as I mentioned previously, a long newspaper with many pages; each "page" is one "screen" worth, analogously. It's like infinite scroll, either vertically or horizontally, where instead of just one item in the list, you have a few. And if we're being really pedantic, Figma users do perfectly fine keeping the context of the content in their minds even in an infinitely pannable canvas. And also, generally newspaper readers do not do what you say, scanning column by column, they instead glance their eyes over all of the headlines then pick which one looks good then they read the article attached to that, it is a free form process.
So again, I will contend that this is not a problem for the average reader. I really cannot see where the difficulty you seem to say lies.