    /* Tall border (taller than our container): */
      border: solid coral;
      border-width: 50px 2px;
      flex: 3;
    }
    .smallItem {
      background: teal;
      height: 20px;
      flex: 1;
    }
    .hidden { overflow: hidden }
  </style>
</head>
<body>
  <div class="flexContainer"><!-- (overflow un-set) -->
    <div class="bigItem"></div>
    <div class="smallItem"></div>
  </div>
  <div class="flexContainer hidden">
    <div class="bigItem"></div>
    <div class="smallItem"></div>
  </div>