Bookmarks Inspector -- collapse vs. toggle

I notice in the Mac manual that the Bookmarks Inspector can only toggle between Document and Project Bookmarks, while the other inspectors use a triangular expand/collapse control to enable their grouped panes to share the space.

Wouldn’t this be a worthy enhancement for Bookmarks as well? Particularly with Project Notes gone, I’d use Project Bookmarks and the editing preview pane a lot more if I didn’t have to hide Document Bookmarks to do so.

Thanks for considering!