[menu] Positioner is repositioned when navigating items

# Bug report

## Current behavior

When the Popup part of the Menu is partly outside the viewport, if you navigate the options via ⬆️ ⬇️ keys, the positioner is repositioned. See attached video.

https://github.com/user-attachments/assets/4e77acad-ca4c-4f20-bc96-dcfb66111e32

## Expected behavior

The Popup should stay in the same place.

## Reproducible example

https://codesandbox.io/p/sandbox/mns5dn?file=/src/App.tsx

## Base UI version

1.0.0-alpha.6