[css-display-4] Initial value of reading-flow

[fantasai]

We had in the past discussed the possibility of the initial value of reading-flow handling some things automatically, e.g. dense packing in Grid or maybe some aspects of Masonry layout. (In that case we'd add an additional keyword to represent strict source ordering.) Opening this issue to track that question.

[rachelandrew]

By automatically, I'm assuming that the default behavior of grid-auto-flow: dense and masonry layout or using would be to follow the 'visual' order, rather than source order.

Given that masonry is new, there wouldn't be an issue there as we could define the behavior from the outset. I outlined some thoughts for the masonry/reading-flow interactions in this issue. Having automatic behavior would I think mean we needed to have the behavior more like flex, and have a masonry-flow value as the behavior of normal in masonry layouts.

Changing the defaults for dense packing might cause a compat issue. My gut feeling tells me that where people have used dense packing they aren't too worried about the ordering, given that you essentially lose control of it anyway, but we probably need to look into it. The default would likely need to follow the grid-auto-flow direction (so reading-flow: grid-rows for grid-auto-flow: row dense).

I think in both cases adding a keyword to reading-flow (perhaps source) to reset it would be needed.