On a PC, the Close and Minimize buttons behave as you would expect for a chat program:
Minimize sends the program to the taskbar as a running program
Close does not actually end your session or close the program, but instead sends it to the system tray
It seems like the Mac version is trying to replicate this functionality, and failing:
Minimize sends the program to the minimize area of the dock - this is fine
Close has this unintended effect: Spark disappears (I assume to the system tray which doesn’t exist). You remain connected to your session, but it seems there is now no way to recover the main window (with chat roster). You can still access the Spark menu by clicking on the main dock icon, but the roster never reappears so you are forced to Quit Spark and reopen.
How it should work (based on other programs on the Mac:
Minimize sends the program to the minimize area of the dock
Close makes the main window disappear from the screen, but does not end your session, and you should be able to bring the main window back by clicking on the main dock icon