How can I make an expanded-entity element collapsible? I would like to do that because I want to also make a diagram showing just the entities and since duplicating work can be avoided by collapsing the above mentioned element type.
From what I can tell there's no setting for making something collapsible, nor can you add it as code (not in the code for the object itself and not in the code for the whole page). I'm not an expert though.
The only thing I can think of, and you've probably thought of this hack, is to add a container (no separator and no fill) for each element that you want to be able to collapse, then put your element inside it. Make the line color of the container "none". Size the element and position it in such a way that the little collapsing UI box is in the position you want relative to your element.
Just be sure that when you move that thing around you're moving the container, not just the element.