current_page is a core part of the Search object again.

Anything that is used by populate shouldn't be in masks.
Search masks are reset for every search.

Otherwise we end up with the constant being modified by any search that may add masks (such as the group enumerators).
GroupEnumeratorsMask is part of the defaults.

Otherwise, it only gets added when populated - which is too late for Search's method_missing. Putting populate before mask calls meant scopes couldn't be chained, so this is the best way forward for the moment.

If there were more masks, then perhaps distinguishing them between pre- and post-populate would be worthwhile. Right now, not the case.