• html：

<div class="box-wrapper">
  <div class="box">
    <div class="item">Home <span class="caret"></span></div>
    <div class="item">Phthon</div>
    <div class="item">React</div>
    <div class="item">Vue <span class="caret"></span></div>
    <div class="item">Java</div>
    <div class="item">Html5</div>
    <div class="item">Javascript</div>
    <div class="item">VuePress</div>
    <div class="item">Bootstrap</div>
    <div class="item">Jquery</div>
    <div class="item">Home</div>
    <div class="item">Order</div>
    <div class="item">Session</div>
    <div class="item">Preety</div>
    <div class="item">Java</div>
    <div class="item">Html5</div>
    <div class="item">Javascript</div>
    <div class="item">VuePress</div>
    <div class="item">Bootstrap</div>
    <div class="item">Jquery</div>
  </div>
</div>
 

• scss:

.box-wrapper {
  width: 100px;
  background: #009688;
  position: fixed;
  top: 0;
  left: 0;
  bottom: 0;
  overflow: hidden;
  box-shadow: 0 0 10px 0 rgba(0, 0, 0, .8);
}
.box {
  height: 100%;
  width: calc(100% + 7px);
  overflow-y: auto;
  text-align: center;
  color: #f2f2f2;
}
.box .item {
  height: 40px;
  line-height: 40px;
  cursor: pointer;
  position: relative;
   &:hover {
    background: #ed686f;
    color: #fff;
     .caret {
       transform: rotateZ(180deg);
     }
   }
   & .caret {
    width: 0;
    heihgt: 0;
    border-top: 4px solid transparent;
    border-bottom: 4px solid transparent;
    border-left: 4px solid #f2f2f2;
    display: inline-block;
    position: absolute;
    top: 17px;
    right: 10px;
    transition: all .2s;
   }
}
 

• 效果展示：

最後總結：其中的原理是外層限制了寬度並且設定超出隱藏，然而我們在內層寬度增加17px,滾動條則顯示在增加的17px裡面。所以有滾動條的時候就不會顯示出來。～～