[Combobox] Provide mechanism for responding to position changes in dropdown

[Yonava]

Feature request

Summary

A mechanism enabling subcomponents such as Trigger to be styled based on where Positioner is placing the dropdown menu.

Motivation

In the case below, the trigger (in red) has border radius that has no clean way of responding to updates from position change. This feature would allow listening for, or taking control of the Positioner for cases like these.

Screen.Recording.2025-12-05.at.12.22.44.AM.mov

[mnajdova]

There is a data-side prop available in the popup that indicates which side of the trigger it is being rendered, you can use that for styling, see the API reference here https://base-ui.com/react/components/combobox#popup

[atomiks]

Looks like you're referring to the side state being available on Trigger (not Popup, to change its border-radius). It's available on Input but not Trigger, but there's no reason it can't also be set there