I too, don't like the fact that using drag and drop means I have to retouch- and re-drag the thumbs from the center position every time I accidentally drop it.
(If I learn how to do the thing I asked on previous post, I would change the touch control into calculating angles between "Control" and "Touch" right away)
I opt using "is not dragging" over "on drop" in order to have the thumbs position on the center of the touch pad on the start of the layout, and in case that it was dislocated from the touch pad for whatever reason.
I always wonder though, if an event is always true, does it makes the event always firing?
for example as you say, when I use "when thumb is not dragging, set position of thumb to control", does the construct doing work reposition thumb to control every tick, taxing performance?