r2c/user/themes/r2c/scss/template/_section.scss

  #page{
    height: 100%;
    .navbar{
      width: 100%;
      height: 50px;
    }
    #start{
      width: 100%;
      height: calc(100% - 50px);
      // display: grid;
      //
      .blocs{
        height: calc(100vh/2 - 25px);
        overflow-y: auto;
        &:nth-child(1){
          background: red;
        }
        &:nth-child(2){
          background: pink;
        }
        &:nth-child(3){
          background: brown;
        }
        &:nth-child(4){
          background: blue;
        }
        &:nth-child(5){
          background: green;
        }
        &:nth-child(6){
          background: yellow;
        }
      }
    }
  }

.top, .bottom{
  display: inline-grid;
    grid-template-columns: auto auto auto;
}
.handler{
  width: 50px;
  height: 50px;
  border-radius: 25px;
  background: red;
  position: absolute;
  left : calc((100%/3) - 25px);
  top: 50%;
  z-index: 999;
}