thank you very much, bear. However, my menu items' contents and width, length are dynamicly changing so I am afraid the way you mentioned may not be able to apply.
Also, interestingly, I found that this mouse-over drop-down menu will be blocked only by <select> html element on that form. It can cover input fields and textfields.
Is there any difference between <select> element and regular <input> element on the same form ?