I've found strange collision when you open window on full-screen and go to other application so that full-screen is on the back, then you have tray icon, then when you press on tray icon image just disappears. I've spend some long hours searching the problem that cause this, when discovered that full-screen mode overlaps icon tray and it just being redrawn by QPainter of MainWindow. This is quite strange problem, so I've dig more and more and found out that hiding Mac OS top menu do the same to the tray icon. It looks like when you hide the menu and in same time try to show IconTray it gives you a punch, so nothing could probably solve this problem. In my case I've changed the flow of the application, so now full-screen window is modal, when you exit full-screen mode it hide main window and tray icon works fine. Conclusion: NO QSystemTrayIcon and showFullScreen at same time
One idea at the time