+
 Foundation = require 'art-foundation'
 Atomic = require 'art-atomic'
 {EventEpoch} = require 'art-events'

    unless @_validScrollPositionCheckScheduled
      @_validScrollPositionCheckScheduled = true
      timeout 250, =>
        if @_validScrollPositionCheckScheduled && !@_gestureActive
          @animateToValidScrollPosition()
        @_validScrollPositionCheckScheduled = false


    scrolledPastStart = mainChildrenOffset >= 0
    scrolled          = @_scrollPosition != _scrollPosition

    if Math.abs(_scrollPosition - @boundSp _scrollPosition) > 1/256
      @scheduleValidScrollPositionCheck()

    # update _tracking
