The GNOME Shell status tray is an interesting implementation written in JavaScript. Consequently, it is approachable to learn, write extensions for, and modify. However, one issue that bothered me was focus stealing.
When clicking on a status tray icon and moving the mouse cursor diagonally down toward the open menu, focus would inadvertently switch to the next adjacent tray icon. This was rather jarring, exacerbated by the gap between the panel icon and the popover menu.
In Ubuntu’s Unity, this behavior is largely absent because indicator icons are spaced differently, significantly reducing accidental applet switches when moving diagonally. It is a thoughtful touch in UI ergonomics (Fitts’s Law in practice).
Update: GNOME Shell later enlarged and tightened the menu hit targets, resolving this problem.