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HTML

<h1>How to get the <code>.bar</code> container to match the height of the overflowed <code>.foo</code> div using <code>flex</code>?</h1>

<input type="radio" id="flex" name="solutions" checked="checked"><label for="flex">Flex solution?</label>

<input type="radio" id="grid" name="solutions"><label for="grid">Grid solution</label>

<input type="radio" id="table-cell" name="solutions"><label for="table-cell">Table-cell solution</label>

<div class="outer">
  <div class="col foo">
    foo
  </div>
  <div class="col bar">
    bar
  </div>
</div>

SCSS

* {
  box-sizing: border-box;
}

#flex:checked {
  ~ .outer {
    display: flex;
    align-items: stretch; /* the default, but placed here anyway */
  }
}

#grid:checked {
  ~ .outer {
    display: grid;
    grid-template-columns: 30px 1fr;
  }
}

#table-cell:checked {
  ~ .outer {
    .col {
      display: table-cell;
    }
  }
}



.outer {
  border: 2px solid red;
  height: 300px;
  width: 200px;
  overflow: auto;
}

.col.foo {
  min-width: 30px;
  height: 900px;
  border: 2px solid blue;
  min-height: 0;
  background: rgba(0,0,255,0.1);
}

.col.bar {
  width: 100%;
  border: 2px solid green;
  background: rgba(0,255,0,0.1);
}