When I first upgraded my Filemaker Pro Advanced to version 17 it had a check list of options with regard to window. I didn't understand the specifics so I experimented by turning them all off. The result was there was no X or other options on the floating windows...which was a problem.
After a time, however they magically reappeared.
The actual problem is, is there a way that when a user presses the X on the Windows control it will kick off a trigger? I haven't found it in the documentation on how to do that or on how to get to the menu that includes the close, etc.