[popups] Allow Positioner parts to be omitted for native CSS anchoring

[atomiks]

When trying to use native CSS anchor positioning by simply omitting the Positioner part to position Popup with CSS anchor, an error is thrown:

Base UI: PopoverPositionerContext is missing. PopoverPositioner parts must be placed within <Popover.Positioner>.

Native CSS Anchoring doesn't support Arrow yet, but it can optionally consume the context instead of throwing for future use. Popup doesn't need to consume the positioner context at all (FloatingFocusManager can work with FloatingRootContext from the root instead).

This way, the Floating UI logic doesn't need to be included in the bundle once CSS anchoring is widespread if they just omit the Positioner part in their JSX.

[vladmoroz]

If it's going down this route, it'd be great to have a first-class way of creating your own custom positioner in general.

In Paper we implemented this with extra Floating UI logic on top of the existing Positioner part, which did require a bit of strong-handing:

Screen.Recording.2025-04-02.at.10.30.26.mov

I'm really happy that was still achievable though

[atomiks]

@vladmoroz any ideas for what the API could look like/how you achieved it externally? I'm guessing you used the anchor prop with a VirtualElement, and added custom dragging functionality while the collision detection worked as normal?