向下滚动时隐藏透明固定导航栏下的内容

the problem: I have transparent fixed navbar with some gap (margin-top) and below content, which is located under the navbar while scrolling down globally. The problem is that navbar is transparent and the background of the page is a dynamic slideshow of different images so I can't use z-index and hide it by changing background color or put image same as background..the problem: I have transparent fixed navbar wi