gistfile1.js

jQuery(document).ready(function($){

  var headerOffset = $(".main-header").height()
    , blockMenu = $(".block-menu")
    , scrolled
    , isBelow = false
    , isCurrentlyBelow = false

  $(window).scroll(function () {
    scrolled = window.pageYOffset || document.documentElement.scrollTop || document.body.scrollTop || 0
    isCurrentlyBelow = scrolled > headerOffset
    
    if(isCurrentlyBelow == isBelow) return
    
    if(isCurrentlyBelow) {
      blockMenu.addClass("is-mini").slideDown("slow")
      isBelow = true
    } else {
      blockMenu.removeClass("is-mini")
      isBelow = false
    }

  })

})