.sidebar {

  padding: 10px;

  background: #e34122;

  flex: 1 1 auto;

  /* Uncomment the line below to see the fix in action */

  /*  min-width: 0; */

}

​

.flex-container {

  display: flex;

  gap: 10px;

  width: 400px;

  font-family: arial;

  font-size: 14px;

  border: 1px dotted blue;

  gap: 10px;

  padding: 10px;

}

​

.main-column {

  flex: 1 0 300px;

  padding: 10px;

  background: #eee;

}

​

span {

  display: inline-block;

  width: 400px;

}