问题现象如题，下面是测试代码和运行结果：
var sub = plus.webview.create('layer-subview.html', 'layer-subview-1', {
left:20,
width: 200,
top: 20,
height: 200
});
sub.show();
plus.webview.currentWebview().append(sub);
var sub = plus.webview.create('layer-subview.html', 'layer-subview-1', {
background: '#008800', // 只增加了这一项
left:20,
width: 200,
top: 20,
height: 200
});
sub.show();
plus.webview.currentWebview().append(sub);
