This may be the issue you are experiencing as this happened to me yesterday. When you move your cursor to the search function, simply start typing your search. When your cursor moves off the search tab, the box disappears.
I typically use Microsoft Edge as my browser for this site. It has come a long way. Other browsers may react differently.
Any flames thrown my way for my choice of browser will be met with Monty Python style taunting in my worst French accent. 
