[c-smile]

This seems to be modelled after Sciter's popups that existed 10 or so years.

anchorElement.popup(elementToPopup, options) - https://docs.sciter.com/docs/DOM/Element/#popup

In Sciter popup element is always associated with its anchor element and appears relative to it.

CSS was expanded to support popups: :popup state flag/selector is "on" the popup element and :owns-popup is "on" on popup anchor element when the popup is shown. Also several CSS properties: popup-position, popup-anchor-reference-point, popup-reference-point, popup-animation.

Sciter has built-in JSX and so element.popup() accepts JSX that creates popup DOM element on demand, for example this

   button.onclick = function() {
      button.popup(<select type="list">
        <option>A</option>
        <option>B</option>
        <option>C</option>
      </select>)
   }

will popup selectable list as dropdown popup.