This problem occurs when you do not have a doc type set for your web pages. A doc type tells browsers how to render your pages to your site visitors.
To correct this issue add the following line of code to the very first line of your HTML on all your site pages:
You can confirm this is placed correctly by viewing the source of your webpage and checking for that line as the very first line drawn. If not placed correctly then your site will not render correctly.
This fix may also solve other display issues you may experience within your website.