Is there any canonical way to constrain locators either continuously (e.g. to lie on the graph of a function) or discretely (snap to grid leaps to mind)? The documentation seems a little sketchy on the subject...
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Dynamic
. $\endgroup$RegionNearest[]
for this answer. If you need locators to be constrained to a function, see this (a similar approach should work for parametrically-defined curves). $\endgroup$