Ability to set `Popup` part widths to the widths of their Triggers

[mxmason]

Feature request

Summary

As it says on the tin, I'd love the ability to ensure that Popup parts have the same width as their triggers.

Motivation

Menu components (<select>s, aria menus, and the like) are often rendered with the same width as their triggers. This can be achieved right now with CSS, but only by setting an exact width on the trigger element and then repeating that number on the Popup, as I've done in this example.

For cases where it isn't possible or desirable to have a fixed width like this, it would be nice to be able to programmatically retrieve the width of the Trigger part, then set that width (or min-width, etc.) of the Popup.

Edit to add: the CSS workaround only works with width and not min-width, meaning that items can't grow past the width of the trigger.

[mj12albert]

Menu.Positioner (and Positioners in general) provides --anchor-height and --anchor-width CSS variables: https://base-ui.com/react/components/menu#positioner

I think that's what you're looking for ~ @mxmason