[css-align-3][css-position-3][css-anchor-position-1] Allow unsafe endward overflow in scrollable areas?

[fantasai]

In #10316 we decided to try having abspos boxes unsafe-overflow their IMCB, but try to stay within the containing block itself (while avoiding the unscrollable region). Avoiding overflow of the containing block makes sense, but it's maybe not as critical as avoiding overlap with the anchor when the containing block is scrollable.

Should we adjust the safety rules to not avoid overflow on the end side of a scrollable container?