[Select] Move `alignItemToTrigger` from `Root` to `Positioner`

[atomiks]

Bug report

The previous implementation required the prop to be on the Root, but this is no longer the case with the latest one, as it can exist on Positioner instead which makes more sense in the API.

@DiegoAndai @aarongarciah you mentioned possibly changing the name, which we could also do simultaneously here.

An actual proper description of the behavior is more like:

<Select.Positioner alignSoThatSelectedItemTextIsOverTriggerValueText>

Which is obviously unrealistically verbose, but is more accurate as the prop aligns the Positioner element, not the Item element, so the selected item text inside of it visually appears directly aligned with the Trigger Value text.

It also disables the other anchoring-related props and enters a "special positioning mode"

<>
  <Select.Positioner align="selected" />
  <Select.Positioner overlayTriggerValue />
  <Select.Positioner alignToTrigger />
  <Select.Positioner anchorMode="selected-item|trigger" />
  <Select.Positioner positioningMode="selected-item|trigger" />
</>

[colmtuite]

Why change the name @aarongarciah?

[aarongarciah]

@colmtuite I can't find the original discussion, but IIRC, it was just a matter of keeping it shorter. There's probably no need to include "item" in the prop name. It probably makes sense to change it to something else since the positioner is what gets aligned, not the items.

@atomiks which props get disabled when entering this "special positioning mode"?

From the options listed, I think these could work, but I need to look deeper into this:

• <Select.Positioner align="selected-item" />
• <Select.Positioner anchorMode="selected-item|trigger" />

positionMethod="absolute|fixed" already exists so positioningMode="selected-item|trigger" would be confusing.

Q: When using alignItemToTrigger={true}, does the align="stard|center|end" prop have an effect when there's no room for the positioner to fit the viewport and it goes into "regular" positioning mode? See attached video. If align still has an effect in this scenario, align="selected" probably wouldn't make sense?

Kapture.2025-04-14.at.16.13.53.mp4