<view class='test'>
      <view class='left'>
      </view>
      <view class='right'>
      </view>
    </view>

.test {
  display: flex;
  flex: 1;
  justify-content: flex-start;
  align-items: center;
  flex-direction: row;
}


.left {
  flex: 1;
  width: 40px;
  background-color: red;
}
 
.right {
  flex: 1;
  height: 400px;
  background-color: #3A7EEE;
}

左边的容器没有高度，如果设置成

.left {
  height: 100%;
  width: 40px;
  background-color: red;
}

为啥红色不自动占满容器高度。