systray hangs if application using the tray exits while context menu is open
I got this with the blueman applet in the systray. If there's no existing bluetooth adapter it could detect when bluetooth is activated using the applet, it force closes itself. If at that time, the context menu on it is open, the systray becomes unusable. Any mouse click on it is not registered by the application. Mouse over events still work though. Screenshot shows what that looks like (no mouse over shown there though, I did not bother showing that particular thing).
xfce4-panel 4.16.0