问题：横向铺满换行，换行之后，第一行和第二行中间的空白是怎么回事？

重现代码

.main {

position: fixed;

height: 100%;

width: 100%;

display:flex;

}

wxss:

.scrollX {

width: 100%;

height: 100%;

display: flex;

flex-wrap:wrap;

flex-direction:row;

}

.back-color{

width: 30%;

height: 25%;

background-color: red;

border: 1rpx solid rebeccapurple

}

<view class=‘main’>

<view class=‘scrollX’>

<view class=‘back-color’>

</view>

<view class=‘back-color’>

</view>

<view class=‘back-color’>

</view>

<view class=‘back-color’>

</view>

</view>

</view>