position: absolute;

.recent-contac {
  position: absolute;
  overflow: auto;
  overflow-x: hidden;
  top: -2px;
  bottom: 0;
  -webkit-user-select: none;

  background-color: yellowgreen;
}

 fixed：

模态窗:

@mixin  fm {
    position: fixed;
    top: 0;
    bottom: 0;
    left: 0;
    right: 0; 
    z-index: 3000;
}

 

.system-settings {
    @include fm;
}

.settings {

    @include center(600px,
    475px);
    background: #fff;
    -webkit-user-select: none;
    user-select: none;
    border: 1px solid #ddd;
    box-sizing: border-box;
    box-shadow: #757b7f 0 0 5px;
}