CSS双层滚动，自适应适配

.tab-content {
  position: absolute;
  top: 0;
  left: 0;
  right: 0;
  bottom: 0;
  overflow-y: auto;
  .custom-scroll();
}

.wrap {
  min-height: 100%;
  .a {
    /* ... */
  }
  .b {
    position: relative;
    .b1 {
      position: absolute;
      top: 0;
      left: 0;
      right: 0;
      bottom: 0;
      overflow-y: auto;
      .custom-scroll();
    }
  }
}