cover-view 不能覆盖 web-view

想在web view控件上放一个小按钮，用了cover-view不能覆盖在web-view上面

<template>  
    <view>  
        <web-view src="https://www.baidu.com/">  
            <cover-view class="tool-bar">  
                <cover-image src="../../static/appLogo.png" class="nav-bg"></cover-image>  
                <cover-view class="tool-btn">  
                    <cover-view>我的</cover-view>  
                    <cover-view>订单</cover-view>  
                </cover-view>  
            </cover-view>  
        </web-view>  
    </view>  
</template>  

<style>  
    .tool-bar {  
        color: #fff;  
        position: fixed;  
        bottom: 13%;  
        right: 0rpx;  
        width: 136rpx;  
        height: 236rpx;  
        border-radius: 8rpx;  
        z-index: 999999;  
        box-shadow: 0px 0px 15px 4px  rgba(0,0,0,0.15);  

        .nav-bg {  
            width: 136rpx;  
            height: 236rpx;  
            position: absolute;  
            left: 0;  
            top: 0;  
            z-index: -1;  
            border-radius: 8rpx;  
        }  

        .tool-btn {  
            width: 100rpx;  
            height: 200rpx;  
            position: absolute;  
            top: 18rpx;  
            left: 18rpx;  
            display: flex;  
            align-items: center;  
            justify-content: center;  
            flex-wrap: wrap;  
        }  
    }  
</style>