下拉菜单没有显示？设置为溢出时右侧显示的空白区域：可见

I'm putting up a site on Tumblr for my online store. Thing is, I can't get the dropdown menu to show up when I put "overflow:hidden" on the #topmenu part. When I set it to "overflow:visible", I get a huge chunk of white space to the right of the webpage. The white space isn't obvious when I'm using my laptop but it is when I use my phone.I'm putting up a site on Tumblr for my online s