Alignment has a concept of "safety", which is, we try to bias alignment in a direction that won't result in clipping the content. For unsafe centering, we don't care; but for safe centering we overflow away from the unscrollable region.

For anchor positioning (and absolute positioning in general to be fair), there's two containers that are of interest: the inset-modified containing block, and the actual containing block.

Should the default behavior and/or the safe behavior of an overflowing alignment bias towards staying within the containing block if possible, before finally overflowing the end/end edges?