按菜单 - 如何推动身体

I have a menu that opens to the right. My issue is that I want the body to resize after I have opened the menu. The menu is 300px wide and I want the body to resize to take up the remainder of the screen. I saw a few examples, but all they do is shift the body so part of it is not visible and off screen. You can see my example I have a menu that opens to the right. My issue