<ul class="menu">
<!--  (li.item>((.item-header>lipsum1)+(.item-body>lipsum5)+(.item-aside>lipsum10)))*10  -->
 <li class="item">
   <div class="item-header">Lorem.</div>
   <div class="item-body">Lorem ipsum dolor sit amet.</div>
   <div class="item-aside">Lorem ipsum dolor sit amet, consectetur adipisicing elit. Fugiat, vitae.</div>
 </li>
 <li class="item">
   <div class="item-header">Nesciunt.</div>
   <div class="item-body">Sit doloremque repellat natus libero.</div>
   <div class="item-aside">Pariatur quibusdam voluptas, vero accusamus itaque. Neque magni autem sunt.</div>
 </li>
 <li class="item">
   <div class="item-header">Animi!</div>
   <div class="item-body">Similique voluptas, sint quam eligendi.</div>
   <div class="item-aside">Unde repudiandae, mollitia voluptatum similique repellendus eum. Ut, quae! Deleniti.</div>
 </li>
 <li class="item">
   <div class="item-header">Fugiat?</div>
   <div class="item-body">Ducimus eaque nulla vitae numquam.</div>
   <div class="item-aside">Ut saepe vel eius velit necessitatibus odit quia nisi cupiditate!</div>
 </li>
 <li class="item">
   <div class="item-header">Sed?</div>
   <div class="item-body">Molestiae quas placeat ad autem?</div>
   <div class="item-aside">Quibusdam labore magni laboriosam quas repellat ullam, in sequi rem.</div>
 </li>
 <li class="item">
   <div class="item-header">Libero.</div>
   <div class="item-body">Id reiciendis nihil laboriosam dolor?</div>
   <div class="item-aside">Laborum praesentium recusandae, aliquam iste ad! Nam ipsa voluptatum laborum?</div>
 </li>
 <li class="item">
   <div class="item-header">Porro.</div>
   <div class="item-body">Nobis quae aut ullam error.</div>
   <div class="item-aside">Voluptate iure, quos tempora dolor officia illum voluptatum praesentium ducimus.</div>
 </li>
 <li class="item">
   <div class="item-header">Dolor.</div>
   <div class="item-body">Perferendis ad officia, enim doloribus.</div>
   <div class="item-aside">Minus sed rem vel commodi eos sint hic architecto consequatur.</div>
 </li>
 <li class="item">
   <div class="item-header">In?</div>
   <div class="item-body">Eum iste labore quo inventore.</div>
   <div class="item-aside">Ratione asperiores autem deleniti iste rerum nesciunt iusto quos, quidem.</div>
 </li>
 <li class="item">
   <div class="item-header">Amet.</div>
   <div class="item-body">Ex nam hic temporibus quaerat.</div>
   <div class="item-aside">Aliquid commodi nam id tempora, placeat quo, ex cupiditate alias.</div>
 </li>
</ul>
* {
  box-sizing: border-box;
}

.menu {
  margin: 15px 10%;
}

.item {
  display: flex;
  position: relative;
  margin-bottom: 15px;
  line-height: 1.6;
  word-wrap: break-word;
}

.item-header {
  width: 2 / 12 * 100%;
  padding: 0 10px;
  text-align: right;
  font-weight: bold;
}

.item-body {
  flex: 1;
  width: 10 / 12 * 100%;
  margin-right: 4 / 12 * 100%;
  padding: 10px;
  background-color: #fafafa;
  
  &:hover + .item-aside {
    z-index: 1;
    opacity: 1;
  }
}

.item-aside {
  width: 4 / 12 * 100%;
  position: absolute;
  z-index: -1;
  top: 0;
  right: 0;
  padding-left: 10px * 2 + 1px;
  padding-right: 10px;
  opacity: 0;
  transition: all 400ms;
  
  // left line
  &::after {
    content: '';
    position: absolute;
    top: 0;
    bottom: 0;
    left: 10px;
    border-left: 1px #ddd solid;
  }
  
  &:hover {
    z-index: 1;
    opacity: 1;
  }
}

External CSS

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External JavaScript

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