Support Subgrid layout

[nicoburns]

What problem does this solve or what need does it fill?

Subgrid layout is useful when creating nested grids.

What solution would you like?

Implement support for grid-template-columns: subgrid and grid-template-rows: subgrid in Taffy:

• Subgrid layout is defined in the CSS Grid Level 2 specification https://www.w3.org/TR/css-grid-2/

Additional context

Much like block layout, subgrid layout requires access to non-direct descendants of a node, so some design work will be required to create an API that allows for this.

[jrmoulton]

I'm really interested in having subgrid.

I'm definitely not qualified but I can work on this if no one else is.

Now that block layout has been merged, is there still design work that needs to be done?

@nicoburns

[nicoburns]

@jrmoulton

Now that block layout has been merged, is there still design work that needs to be done?

Block layout isn't really using this yet, but there is a design for this now. The trick is that each algorithm now takes it's own trait as a parameter. So we can add algorithm-specific methods to that trait, one of which can be to get access to items in a subgridded axis.