Custom gutter boundaries

What do you think of adding API support for custom boundary positions? I.e. instead of the gutters starting at 0 and `contentSize - viewportSize`, they could start at user defined positions.

I'm thinking this could be useful for "pull to refresh" functionality (when the scroll is below, for instance, -100, user could lock both boundaries at -100 such that the gutter remains showing, do the refresh, then reset the boundaries as they were before)

Probably relates to #2 as far as API is concerned? Though not exactly sure what you meant there