Style parts of <sl-popup> within <sl-dropdown> (exportparts)

[devopvoid]

Describe the bug

Referring to the issue #858, which has been partially fixed, for <sl-popup> only. It is not currently possible to style the popup part inside a <sl-dropdown>, as this introduces a new level of #shadow-root.

The tooltip component is exporting the popup and arrow names, which allows you to select and style them in nested shadow trees.

It would be helpful if exportparts were introduced for the dropdown as well.

<sl-dropdown>
  #shadow-root
    <sl-popup part="base" exportparts="popup:base__popup, arrow:base__arrow">
      #shadow-root
        ...
        <div part="popup" ... >
        ...

[claviska]

This seems like a fair request.

If anyone wants to take a stab at it, you'll need to export the respective parts and update the docs with @csspart attributes. (Tooltip is a good reference for how to approach it.)