moveend and not zoomend: zooming never changes a scale→zoom conversion, only moving
in latitude does. Called once the map exists; resets the drift tracking so a re-created
map starts from a clean slate rather than inheriting the previous one's latitude.
Binds the latitude watcher to the map.
moveendand notzoomend: zooming never changes a scale→zoom conversion, only moving in latitude does. Called once the map exists; resets the drift tracking so a re-created map starts from a clean slate rather than inheriting the previous one's latitude.