SOURCE

<div class="wrapper">
	<div class="tools">
		<select name="mode" id="pet-select">
            <option value="1">go to dart</option>
             <option value="1">create dart model</option>
        </select>
	    <button>转换</button>
  </div>
  <div class="outPanel">
      <textarea placeholder="输入源代码"></textarea>
    <div class="right">
      转换结果
    </div>
  </div>
</div>

body,
html {
    margin: 0;
}

.wrapper {
    display: flex;
    flex-direction: column;
    min-height: 100vh;
}

.tools {
    line-height: 32px;
    display: flex;
    border-bottom: 1px solid #ccc;
    padding: 10px;
}

.tools .select {
    background: transparent;
    color: #ccc;
}

.tools button {
    margin-left: 10px;
    background: transparent;
    color: #ccc;
    border: 1px solid #ccc;
}

.outPanel {
    flex: 1;
    padding: 10px;
    display: flex;
}

.outPanel textarea {
    flex: 1;
    border: 1px solid #ccc;
    padding: 10px;
    border-radius: 4px;
    background: transparent;
    color: #ccc;
}

.outPanel .right {
    flex: 1;
    margin-left: 10px;
    border: 1px solid #ccc;
    padding: 10px;
    border-radius: 4px;
}