In Preferences in the section browsing activate "browseWithDragNDrop"
and "dragNDropWithAnimation". Open browser and select a class in any
class category. Now select any method category and try to move a method
to any other method category whithout selecting the method first. This
results in a walkback because moving a method #nil. Now deselect any
method category, so that none is selected, and again try to move any
method to any method category (without selecting the method first). This
time a "new" empty method named Definition appears in the list of
methods.

I run into this more often when using a touchpad on a notebook where you
can "tip" on the pad to click left and "tip+move" to press left and
move.

Alex