Implement CSS Grid

[nicoburns]

Objective

Fixes #204

Feedback wanted

Nothing in particular just yet. This PR is still very incomplete. Just putting it up early to track progress.

[TimJentzsch]

FYI, if your work is still in progress it's good practice to mark the PR as draft: GitHub Docs

This prevents merging of the PR before it's ready.

You should be able to mark it as draft retroactively in the sidebar on the right, if IIRC.

[alice-i-cecile]

As a heads up @nicoburns, I'd like to merge this relatively aggressively, then iterating on it in follow-up PRs before the next major version. Great work so far!

It's a much easier workflow for collaboration than trying to get it perfect in the very first PR.

[nicoburns]

@alice-i-cecile Good to know! I can break this up into smaller PRs if that would be helpful? And maybe add feature flags for flexbox and grid (flexbox defaulting to on, grid to off for now)?

However this PR builds on top of #202 (as it already much of the work of nicely factoring out the existing flexbox implementation into it's own module). I'm not sure what the status on that PR is, but currently this one would be blocked on that one being merged.

[nicoburns]

@alice-i-cecile

I believe I have addressed all of your concerns/requests (in particular I've filled in the release notes). I have also:

• Implemented absolute and hidden positioning
• Fixed support for children with margins (including absolutely positioned children).
• Added a whole bunch of tests and fixes. We're up to 100 gentests for grid now (compared to 277 for Flexbox). Still a lot more testing to be done, but I think this is a good start.
• Updated CI to run tests both with and without the grid feature flag. Unfortunately the only way I could get this to work was remove it from the "taffy" section in Cargo.toml, which means that my editor doesn't seem to pick up grid related files by default, cargo test doesn't include grid tests by default, and clippy doesn't lint grid files by default (you have to specify --features experimental_grid). Not sure if there is anything we can do about this. For now I've been adding it back it temporarily but not committing it. Perhaps we should make it a default features? Not sure.

As always, progress is being tracked over at #204. My suggestion is that we try to get this PR merged ASAP, but that we keep the issue open and continue to use it to track overall progress on Grid. I imagine the release notes might need some work, but I would like to suggest we start a new release issue/PR and don't block this PR on that.

[alice-i-cecile]

Awesome write-up! That was my last blocking request: I think that merging and iterating is the way to go now.