body {
    display: flex;
    flex-direction: column;
    overflow: hidden;
    width: 600px;
    height: 600px;
    box-sizing: border-box;
    margin: 0;
    padding: 8px;
  }
  
  .btn-tab {
    position: relative;
    top: 1px;
    border: 1px solid #aaa;
    border-top-left-radius: 5px;
    border-top-right-radius: 5px;
    background: #fff;
    cursor: pointer;
    outline: none;
    transition: all 0.4s;
  }
  
  .btn-tab:active,
  .btn-tab:focus {
    border-top-color: #5B9DD9;
    box-shadow: 0px -1px 0px 0px #5B9DD9;
  }
  
  .btn-tab.active {
    border-bottom-color: #fff !important;
  }
  
  .txt {
    flex: 1;
    resize: none;
    padding: 5px;
  }