[css-display-4] reading-flow following grid-auto-flow

[fantasai]

Would it make sense to have a value of reading-flow that follows the grid-auto-flow property, rather than needing to be explicit about rows vs columns?

[tabatkins]

We could theoretically do this, but if it's not the default for Grids (and I don't think it should be), then I don't see too much value in it.

It shouldn't be the default, because grid-auto-flow: row is already the initial value, even if you're not using auto-flow abilities. For a non-auto-flowing Grid, we don't want to try and guess at the author's intent. We unfortunately don't really have a good explicit signal of auto-flow usage (we don't want to rely on the presence of auto-flowing children to determine this; it would mean the reading order of the fixed items could change based on whether the container is empty or not).

So if it's not the default, you have to explicitly set it; does it really help authors to be able to say grid-auto-flow: row; reading-flow: grid-auto-flow; vs grid-auto-flow: row; reading-flow: grid-rows;? If switching the auto-flow direction was a reasonable thing to do, I could see it still being valuable so you didn't have to set the direction in two spots, but I don't think that's the case; the auto-flow direction is basically set as part of the element design. Grid authors already have three grid-* values to choose from; I don't think adding this as a fourth justifies the increased mental load.