I could just be having a brain freeze, but just sharing in case anyone else is similarly going mad... a few times recently in Nautilus I've dragged a method from the fourth pane and dropped it on a protocol in the third pane and the method disappeared from the system. Luckily its easily recovered from Tools > Code Changes, where it shows as deleted. But its weird that a delete could be triggered from the action I (thought I) performed.
I've yet to detect a pattern. Unfortunately its kind of blink-and-you-miss-it. Its not until a second later when selecting the target protocol that the behaviour becomes apparent. This is in 60527-64, Unix built on Jul 20 2017 20:40:36 Compiler: 4.6.3 cheers -ben