Fullscreen GTK Box should only fullscreen webkit2gtk window, but fullscreens whole app instead

Question

I am attempting to make a Webkit2gtk browser in Rust using WebKit2Gtk-rs. When the WebView attempts to go into fullscreen, it makes the entire application fullscreen, including stuff outside the webview. screenshot of application

What I am doing is creating a GTK Box then stuffing the two Gtk Buttons into it, along with the Webkit2Gtk WebView. Main Window -> Gtk Box -> Button1, Button2, WebView. I would like for only that one element, WebView to go fullscreen.

Source code for the relevant file can be found here.