[css-grid-2] Clarify interaction between subgrid and position:relative #7123

The current spec says that subgrid containers should always be stretched, but doesn't say anything about positioning. It does say that the grid itself should be aligned layout wise with the corresponding section of the parent grid.

If I apply position:relative and an offset to a subgrid, what should happen?

Should we offset the subgrid container (thus moving the rendering of the background/border etc) while leaving the grid aligned (such that the grid might be outside of the grid container)? Or should the positioning request be ignored, so that the subgrid container always covers the spanned grid tracks.