vue移动端背景图无法铺满真个屏幕解决方案

<template>
  <div id="registerBox">
    <button class="download">123</button>
    <div class="tip"></div>
    <button>456</button>
  </div>
</template>

export default {
     
    name: 'registerBox',
    data() {
     
        return {
     

        }
    }
}

#registerBox {
     
        background: url('../assets/download.png');
        background-size: 100% 100%;
        height: 100%;
        position: fixed;
        width: 100%;
        button {
     
            margin: auto;
            margin-top: .625rem;
            background-color: #fff;
            outline: none;
            border: 1px solid transparent;
            width: 8.4375rem;
            height: 1.875rem;
            color: #7259FD;
            font-size: .625rem;
        }
        .download {
     
            margin-top: 95%;
        }
        .tip {
     
            font-size: .3125rem;
            color: #fff;
            margin-top: .3125rem;
            .tips {
     
                margin-left: -2rem;
            }
        }
    }