在'a'标记上，悬停时在其上添加高度或填充底部.您的'a'标签可能需要绝对放置，以使其高度不会影响标头的高度.

On the 'a' tag, add a height or padding-bottom to it on hover. Your 'a' tag might need to be positioned absolute so that its height won't affect the height of your header.

类似以下内容

.about-dropdown a:hover {
    padding-bottom: 30px; /*height dependent on the gap you want to fill*/
    position: absolute; 
}