I don't quite understand how it avoids the case for code that pushes lots of stuff to a vector, then shifts its all out, and then does this again—which, to me,

I don't quite understand how it avoids the case for code that pushes lots of stuff to a vector, then shifts its all out, and then does this again—which, to me, doesn't seem like an edge-case at all. Do any languages automatically reduce vector size when elements are removed from it? If this is the first one, then arguably it would be a bit surprising behaviour.