How to make a card layout collapsable?

That's not a feature of a!cardLayout().  You could potentially accomplish a similar effect with a!boxLayout(), though.

Also note: there's nothing keeping you from implementing something like this in a!cardLayout() - manually, for yourself.  You could have a Rich Text Icon in your card somewhere which, when clicked, hides all contents of the card (other than a "reopen" icon).  That wouldn't be very difficult to do, wouldn't require any unusual workarounds, but isn't a built-in feature.