<nav role="navigation" class="priority-nav">
  <ul>
    <li><a href="#">Home</a></li>
    <li><a href="#">About</a></li>
    <li><a href="#">Clients</a></li>
    <li><a href="#">Contact Us</a></li>
    <li><a href="#">Buy Lemons</a></li>
    <li><a href="#">Dance Wildly</a></li>
    <li><a href="#">Go To Bed</a></li>
    <li><a href="#">Donate Millions</a></li>
    <li class="overflow-nav">
      <span class="overflow-nav-title">
        ▼
      </span>
      <ul class="overflow-nav-list"></ul>
    </li>
  </ul>
</nav>  
@import url(https://fonts.googleapis.com/css?family=Source+Sans+Pro);

.priority-nav {
  background: #f06d06;
  height: 40px; // not ideal... do with JS?
  > ul {
    list-style: none;
    > li {
      display: inline-block;
      position: relative;
      &.overflow-nav {
        display: none;
      }
    }
  }
  a {
    text-decoration: none;
    color: white;
    display: inline-block;
    padding: 10px 15px;
  }
  &.resizing {
    overflow: hidden;
  }
}

.overflow-nav-list {
  background: white;
  box-shadow: 0 0 10px rgba(black, 0.35);
  position: absolute;
  top: 95%; // moves to 100% on hover generally
  right: 0;
  opacity: 0;
  visibility: hidden;
  padding: 5px 0;
  width: 200px;
  color: #222;
  a {
    color: black;
    font-weight: bold;
    &:hover,
    &:active {
      background: #f06d06;
      color: white;
    }
  }
  list-style: none;
  z-index: 1;
  right: -20px;
  &.show {
    opacity: 1;
    visibility: visible;
  }
  > li > a,
  .primary > a {
    color: black;
    display: block;
  }
  .primary-last {
    margin-right: 0;
  }
}
.overflow-nav-title {
  cursor: pointer;
  text-align: center;
  background: white;
  color: #222;
  border-radius: 50%;
  font-size: 10px;
  width: 2em;
  height: 2em;
  line-height: 2.2em;
  vertical-align: top;
  display: inline-block;
}

.hide {
  display: none !important;
}
.show-inline-block {
  display: inline-block !important;
}

body {
  font-family: 'Source Sans Pro', sans-serif;
}
View Compiled
var PriorityNav = {

  // used multiple times, so saving.
  menu: $(".priority-nav"),
  allNavElements: $(".priority-nav > ul > li:not('.overflow-nav')"),

  init: function() {
    this.bindUIActions();
    this.setupMenu();
  },

  setupMenu: function() {
    
    // Checking top position of first item (sometimes changes)
    var firstPos = $(".priority-nav > ul > li:first").position();
        
    // Empty collection in which to put menu items to move
    var wrappedElements = $();

    // Used to snag the previous menu item in addition to ones that have wrapped
    var first = true;

    // Loop through all the nav items...
    PriorityNav.allNavElements.each(function(i) {
      
      var el = $(this);

      // ...in which to find wrapped elements
      var pos = el.position();
                  
      if (pos.top !== firstPos.top) {

        // If element is wrapped, add it to set
        wrappedElements = wrappedElements.add(el);

        // Add the previous one too, if first
        if (first) {
          wrappedElements = wrappedElements.add(PriorityNav.allNavElements.eq(i-1));
          first = false;
        }
      }
    });
        
    if (wrappedElements.length) {

      // Clone set before altering
      var newSet = wrappedElements.clone();

      // Hide ones that we're moving
      wrappedElements.addClass("hide");

      // Add wrapped elements to dropdown
      $(".overflow-nav-list").append(newSet);

      // Show new menu
      $(".overflow-nav").addClass("show-inline-block");

      // Make overflow visible again so dropdown can be seen.
      $(".priority-nav").css("overflow", "visible");

    }
  },

  tearDown: function() {
    $(".overflow-nav-list").empty();
    $(".overflow-nav").removeClass("show-inline-block");
    PriorityNav.allNavElements.removeClass("hide");
  },

  bindUIActions: function() {
    $(".overflow-nav-title").on("click", function() {
      $(".overflow-nav-list").toggleClass("show");
    });

    $(window)
      .resize(function() {
        PriorityNav.menu.addClass("resizing");
      })
      .resize(_.debounce(function() {
        PriorityNav.tearDown();
        PriorityNav.setupMenu();
        PriorityNav.menu.removeClass("resizing");
      }, 500));
  }

}

PriorityNav.init();

External CSS

This Pen doesn't use any external CSS resources.

External JavaScript

  1. https://cdnjs.cloudflare.com/ajax/libs/jquery/2.1.3/jquery.min.js
  2. https://cdnjs.cloudflare.com/ajax/libs/lodash.js/3.5.0/lodash.min.js