SOURCE

$(document).ready(function() {
  $(".drop .option").click(function() {
    var val = $(this).attr("data-value"),
        $drop = $(".drop"),
        prevActive = $(".drop .option.active").attr("data-value"),
        options = $(".drop .option").length;
    $drop.find(".option.active").addClass("mini-hack");
    $drop.toggleClass("visible");
    $drop.removeClass("withBG");
    $(this).css("top");
    $drop.toggleClass("opacity");
    $(".mini-hack").removeClass("mini-hack");
    if ($drop.hasClass("visible")) {
      setTimeout(function() {
        $drop.addClass("withBG");
      }, 400 + options*100); 
    }
    triggerAnimation();
    if (val !== "placeholder" || prevActive === "placeholder") {
      $(".drop .option").removeClass("active");
      $(this).addClass("active");
    };
  });
  
  function triggerAnimation() {
    var finalWidth = $(".drop").hasClass("visible") ? 22 : 20;
    $(".drop").css("width", "24em");
    setTimeout(function() {
      $(".drop").css("width", finalWidth + "em");
    }, 400);
  }
});

<div class="drop">
  <div class="option active placeholder" data-value="placeholder">
    Choose wisely
  </div>
  <div class="option" data-value="wow">
    Wow!
  </div>
  <div class="option" data-value="drop">
    So dropdown!
  </div>
  <div class="option" data-value="select">
    Very select!
  </div>
  <div class="option" data-value="custom">
    Much custom!
  </div>
  <div class="option" data-value="animation"> 
    Such animation!
  </div>
</div>

*, *:before, *:after {
  box-sizing: border-box;
  margin: 0;
  padding: 0;
}

body {
  background: #35414a;
}

.drop {
  width: 20em;
  margin: 5em auto;
  font-family: pfs-bold;
  color: #86919a;
  text-transform: uppercase;
  position: relative;
  transition: width 0.5s;
  will-change: width;
}
.drop .option {
  padding: 1em;
  cursor: pointer;
  background-color: #485761;
}
.drop .option:not(.active) {
  display: none;
  opacity: 0;
  transform: translateY(-50%);
}
.drop.visible {
  -webkit-animation: bounce 1s;
          animation: bounce 1s;
  width: 24em;
}
.drop.visible:before, .drop.visible:after {
  border-color: #fff;
}
.drop.visible:before {
  opacity: 0;
}
.drop.visible:after {
  opacity: 1;
}
.drop.visible .option {
  color: #fff;
  display: block;
}
.drop.opacity .option {
  transform: translateZ(0);
  opacity: 1;
}
.drop.opacity .option:nth-child(0) {
  transition: opacity 0.5s 0s, transform 0.5s 0s;
}
.drop.opacity .option:nth-child(1) {
  transition: opacity 0.5s 0.1s, transform 0.5s 0.1s;
}
.drop.opacity .option:nth-child(2) {
  transition: opacity 0.5s 0.2s, transform 0.5s 0.2s;
}
.drop.opacity .option:nth-child(3) {
  transition: opacity 0.5s 0.3s, transform 0.5s 0.3s;
}
.drop.opacity .option:nth-child(4) {
  transition: opacity 0.5s 0.4s, transform 0.5s 0.4s;
}
.drop.opacity .option:nth-child(5) {
  transition: opacity 0.5s 0.5s, transform 0.5s 0.5s;
}
.drop.opacity .option:nth-child(6) {
  transition: opacity 0.5s 0.6s, transform 0.5s 0.6s;
}
.drop.opacity .option:nth-child(7) {
  transition: opacity 0.5s 0.7s, transform 0.5s 0.7s;
}
.drop.opacity .option:nth-child(8) {
  transition: opacity 0.5s 0.8s, transform 0.5s 0.8s;
}
.drop.opacity .option:nth-child(9) {
  transition: opacity 0.5s 0.9s, transform 0.5s 0.9s;
}
.drop.withBG .option {
  transition: background-color 0.1s;
}
.drop.withBG .option:not(.placeholder):hover {
  background-color: #5aafee;
}
.drop.withBG .option:not(.placeholder).active {
  background-color: #5aafee;
}
.drop:after, .drop:before {
  content: "";
  position: absolute;
  top: 1.5em;
  right: 1em;
  width: 0.75em;
  height: 0.75em;
  border: 0.2em solid #86919a;
  transform: rotate(45deg);
  transform-origin: 50% 50%;
  transition: opacity 0.2s;
}
.drop:before {
  border-left: none;
  border-top: none;
  top: 1.2em;
}
.drop:after {
  border-right: none;
  border-bottom: none;
  opacity: 0;
}

.mini-hack {
  opacity: 0;
  transform: translateY(-50%);
}