High level goal: Create a select picker that shows its whole content if there's space, up to a maximum of 300px. It will shrink to a minimum of 100px tall if it gets close to the viewport edges, and it will leave a 15px margin. If there's no space below (considering the min-height), it will flip above the anchor. It never obscures the anchor, unless the window size is so small, there isn't 100px of space above or below the anchor.

The anchors are draggable.