求助：用 Uikit 建一个瀑布流时第一个 box 异常显示

HMTL:

<div class="uk-grid uk-flex uk-flex-item-auto">
        <div class=" uk-grid-medium " ><div class="uk-panel uk-panel-box" style="background:#5d925b;">Box1Box1Box1Box1Box1Box1Box1</div></div>
        <div class=" uk-grid-medium " ><div class="uk-panel uk-panel-box"       style="background:#345233;">Box2Box2Box2Box2Box2Box2Box2Box2Box2Box2Box2Box2Box2Box2</div></div>
        <div class=" uk-grid-medium " ><div class="uk-panel uk-panel-box" style="background:#893742;">Box3Box3Box3Box3Box3Box3Box3Box3Box3Box3</div></div>
        <div class=" uk-grid-medium " ><div class="uk-panel uk-panel-box" style="background:#cc1133;">Box4</div></div>
        <div class=" uk-grid-medium " ><div class="uk-panel uk-panel-box" style="background:#1a4344;">Box5</div></div>
        <div class=" uk-grid-medium " ><div class="uk-panel uk-panel-box" style="background:#443f14;">Box6</div></div>
        <div class=" uk-grid-medium " ><div class="uk-panel uk-panel-box" style="background:#244424;">Box7</div></div>
        <div class=" uk-grid-medium " ><div class="uk-panel uk-panel-box" style="background:#0e0d44;">Box8</div></div>
        <div class=" uk-grid-medium " ><div class="uk-panel uk-panel-box" style="background:#441520;">Box10</div></div>
        <div class=" uk-grid-medium " ><div class="uk-panel uk-panel-box" style="background:#442429;">Box11</div></div>
        <div class=" uk-grid-medium " ><div class="uk-panel uk-panel-box" style="background:#345233;">Box12</div></div>
        <div class=" uk-grid-medium " ><div class="uk-panel uk-panel-box" style="background:#893742;">Box13</div></div>
        <div class=" uk-grid-medium " ><div class="uk-panel uk-panel-box" style="background:#cc1133;">Box14</div></div>
        <div class=" uk-grid-medium " ><div class="uk-panel uk-panel-box" style="background:#1a4344;">Box15</div></div>
        <div class=" uk-grid-medium " ><div class="uk-panel uk-panel-box" style="background:#443f14;">Box16</div></div>
        <div class=" uk-grid-medium " ><div class="uk-panel uk-panel-box" style="background:#244424;">Box17</div></div>
        <div class=" uk-grid-medium " ><div class="uk-panel uk-panel-box" style="background:#0e0d44;">Box18</div></div>
        <div class=" uk-grid-medium " ><div class="uk-panel uk-panel-box" style="background:#2f440e;">Box19</div></div>
        <div class=" uk-grid-medium " ><div class="uk-panel uk-panel-box" style="background:#441520;">Box20</div></div>
    </div>
![Alt test][flex photo]
![Alt test][box1]
![Alt test][box8]