I recently created a demo of the 'ideal' sizing for custom select, and noticed that how anchor positioning reacts to scrolling is inconsistent across browsers and platforms.

The following videos have audio, but you need to unmute them to hear it. Sorry the volume levels are different between the desktop and mobile recordings.

Firefox desktop

Firefox Android

Chrome desktop

Chrome Android

Safari Technology Preview

Are any of these right?

None of them feel right, and it isn't clear to me if any of these are right according to the spec.

I would expect the layout to update on scroll end at the latest. The current behaviours feel really buggy across the board.