Tooltip visible only if systray is in the bottom panel

[capacitor]

I'm using Ubuntu MATE 15.04. The tooltip that shows all the incoming mail and other useful informations when the mouse is over the icon is not visible. As I found here (http://ubuntuforums.org/showthread.php?t=2031846), it becomes visible if I transfer the systray from the top in the bottom panel. When there, it works perfectly, it doesn't even change position depending on the number of lines displayed, as it append with Ubuntu 12.04 (but I think that's an already known bug).