Repeated contour adjustment using trackpad scrolling fails

[olibclarke]

It seems to adjust the contour by a very small amount, then stop adjusting it at all, with entries like this in the log (on a macbook pro):

INFO::       contour level for map 3 is 0.802453 pending 0
INFO::       contour level for map 3 is 0.802453 pending -1
INFO::       contour level for map 3 is 0.798162 pending -1
INFO::       contour level for map 3 is 0.793872 pending -1
INFO::       contour level for map 3 is 0.789581 pending 0
INFO::       contour level for map 3 is 0.789581 pending -1
INFO::       contour level for map 3 is 0.785291 pending -1
INFO::       contour level for map 3 is 0.781000 pending 1
INFO::       contour level for map 3 is 0.785291 pending 1
INFO::       contour level for map 3 is 0.789581 pending 0
INFO::       contour level for map 3 is 0.789581 pending 0
INFO::       contour level for map 3 is 0.789581 pending 0
INFO::       contour level for map 3 is 0.789581 pending 0
INFO::       contour level for map 3 is 0.789581 pending 0
INFO::       contour level for map 3 is 0.789581 pending 0
INFO::       contour level for map 3 is 0.789581 pending 0
INFO::       contour level for map 3 is 0.789581 pending 0
INFO::       contour level for map 3 is 0.789581 pending 0
INFO::       contour level for map 3 is 0.789581 pending 0
INFO::       contour level for map 3 is 0.789581 pending 0

[pemsley]

I have tried adding more debugging. I haven't got the the bottom of it yet.