Reduce excess positioning calls for ComboBox popup. (#11808)

Reduce excess positioning calls for ComboBox popup. (#11808)

- If an open popup is reset to its default position on every update from
the server before getting adjusted again to the actual expected
position, on heavier applications some of those intermediate positions
might get rendered. If the ComboBox is positioned at the right edge and
the popup contents are longer than the input field (i.e. popup should
open to the left, not right) this might cause flickering.
- Setting the default position is only actually needed when the popup is
opened in order to give it a baseline, otherwise it's better to simply
adjust the position if needed.

Continues on #11718