[css3-grid-layout] [css3-layout] default layout of children

Neither Template Layout nor Grid Layout seems to have a very good default
way of handling content. Grid afaict winds up positioning unpositioned
things very randomly, depending on how positioned content is interleaved;
although maybe I'm misreading the algorithm. Template offers the default
slot option, which seems a good idea to me; but the fallback behavior of
dumping things into the first slot doesn't seem ideal.

I don't have a good answer here, but if it's called out as an issue, maybe
someone will come up with a good idea.

IMO layout systems should avoid dataloss by default, which means
   - content doesn't disappear
   - content doesn't stack on top of each other
   - content doesn't overflow
unless you explicitly ask for it.

One possibility is to dump all the remaining content into a new row at the
bottom of the grid.

~fantasai