Hey there,
I’ve inspected the HTML and CSS of that page. If I disable this line, everything goes back to working:
.page-id-270 #masthead {
Height: 10px;
}
I’m still studying CSS myself, but I think what’s happening here is that you’re forcing the height of that section to be 10px, and this prevents the height from increasing when the menu is clicked, and this height increase is what moves the rest of the content down to make way for the menu (I think!).