Div behavior

I’m having a bit of a problem with a div jumping around, but only when viewing an archived page like a tag or category page.

When you click on “search” it is supposed to fill the main div completely, but when clicking on it in one of the above mentioned views, it jumps below the main div and then corrects itself when a second option within the search div is selected. For example, when viewing it in the example link for it not working, and you click on, say, “Categories”, it corrects its positioning.

Example of it working properly:
Single post

Example of it not working properly:
WWW Category

I don’t see any closed divs and the code is the (as far as I can tell) as the code used in single and index view so I’m a bit stumped as to what is causing this behavior.