When moving the mouse rapidly from inside the body of a TriggerField, through the trigger or back in the same way, the following error occurs:
If you go back one level in the call stack, examine the relatedTarget of the event, and it's very strange. It's an anonymous DIV which is apparently a child element of the <input> element! This must be some internal "virtual" element which the Gecko engine uses, but unfortunately, it gets exposed through relatedTarget.
Here you can see the details. The parentNode of the related div is the input element: