Workaround using flex instead:
With sufficient vertical space this popover is center positioned.
In a shorter viewport this pane is scrollable.
I know this looks like it ought to be a dialog. It is a contrived test.
Some text content to fill out the box. Some text content to fill out the box.
Some text content to fill out the box. Some text content to fill out the box.
Some text content to fill out the box. Some text content to fill out the box.
Look for whitespace below in a taller viewport, e.g. in Safari.