backbutton返回键的问题

图片中空白地方式侧边导航栏，当我点击返回键的时候就出现此时的效果。。怎么回事
代码如下：
<script type="text/javascript" charset="utf-8">
mui.init({
subpages: [{
url: 'menu.html',
id: 'menu.html',
styles: {
top: '48px', //mui标题栏默认高度为48px；
bottom: '50px' //默认为0px，可不定义；
}
}]
});
sqlLiteInit();
var menu, showMenu = false,
slideTogether = false;
mui.plusReady(function() {
var ws = plus.webview.currentWebview();
var left = '0%';
if (slideTogether) {
left = '-70%';
}
menu = mui.createWindow({
id: 'offmenu.html',
url: 'offmenu.html',
styles: {
left: left,
width: '40%',
zindex: 9998
}
});
if (menu) {
menu.show('none');
}
ws.setStyle({
zindex: 9999
})

    });  

    document.querySelector('.mui-icon-bars').addEventListener('tap', function() {  
        mui.currentWebview.setStyle({  
            left: showMenu ? '0' : '40%',  
            transition: {  
                duration: 200  
            }  
        });  
        showMenu = !showMenu;  
    });  

    function closeMenu() {  
            var ws = plus.webview.currentWebview();  
            ws.setStyle({  
                left: '0',  
                transition: {  
                    duration: 200  
                }  
            });  
            showMenu = false;  
        }  
        //按返回图标时，关闭菜单webview  
    document.querySelector('.mui-action-back').addEventListener('tap', closeMenu);  
    mui.plusReady(function() {  
        //按返回键时，关闭菜单webview  
        plus.key.addEventListener('backbutton', closeMenu);  

    });  
</script>  

</html>