I'm currently working with a design that contains a small search icon. Clicking or pressing that icon displays the search form, which is hidden by default. The triggered search form covers the main site navigation. (Clicking on the search icon again hides the search form):
This design has been rejected by our UX team because "the search form shouldn't cover up the navigation". That seems like an odd reason to me. If the user clicks on the search icon, their expected next step is to use the search form, not navigate. In fact, I would say that hiding the navigation behind the search form is actually a benefit since it removes clutter that the user has indicated she is not interested in (by clicking the search icon, the user has indicated that they are not interested in using the navigation currently).
Am I missing something here? Is there a hard rule in UX that states you should never hide the main navigation?