[css-grid-2] Clarify interaction between subgrid and position:absolute

[mattwoodrow]

This is somewhat similar to #7123, except for position:absolute instead of position:relative.

If a subgrid is position:absolute should all top/left/right/bottom values be ignored, and we should just stretch and position across the corresponding grid area in the parent? Should we (similar to one option for relative) try to respect these as much as possible while still forcing the width/height to the stretched value (if both left and right are set, what's the precedence between each of these and the stretched width?)?

What happens if a position:absolute subgrid doesn't cover any grid tracks and only covers the padding area of the parent (grid-row: auto / 1)? Should this be listed as an explicit reason that the specified value of subgrid isn't applied?

[tabatkins]

This was technically already defined in CSS Display (abspos forces an IFC, which turns off subgrid), but we clarified it in Position, and added a reminder in Grid 2.

[mattwoodrow]

Ah right, totally missed that, so thanks for adding the reminder.

There are a lot of WPTs testing that position:absolute works with subgrid, I'll fix this in WebKit and remove those tests (and add new ones to test that various IFC cases aren't subgrids).

[tabatkins]

Ah, thanks, that would be wonderful.