Can't drag website (external bookmark) to create copyholder

Not sure if this is a bug or intentional: I know that when dragging a file to the editor’s header, and holding the OPT key (to get the green + sign to show), a copy holder window opens with that document. But when I try to drag an external bookmark from the project bookmarks pane into the editor header, the OPT key does not trigger the same functionality.

I can create a copyholder from an internal file, and from there, I can drag that bookmark into the copyholder’s title bar to get it to load that web page, so I know the copyholder is capable of rendering the page.

New discovery: When I do this for a URL that happens to play music, and I accidentally close the copyholder by switching to one of the built-in layouts, the music keeps playing. This seems to me to be a problem with data not being cleaned up when an editor/copyholder is closed.