ul#main_content {
  display: table;
  position: relative;
  margin-top: 40px;
  width: 100%;
  height: calc(100vh - 142px);
  box-shadow: 0px 0px 20px rgba(0, 0, 0, 0.5) inset;
}
li#panel_1,
li#panel_2,
li#panel_3 {
  display: table-cell;
  margin: 0px;
  padding: 0px;
  list-style-type:none;
  position: relative;
  vertical-align: top;
}
li#panel_2 {
  height: 100%;
  width: 20px !important;
}