macos: make non-native fullscreen windows not resizeable (#2903)

I've noticed that I can resize a non-native fullscreen window when
selecting text to copy. I think it shouldn't be resizeable.

macos/Sources/Helpers/Fullscreen.swift

@@ -198,6 +198,10 @@ class NonNativeFullscreen: FullscreenBase, FullscreenStyle {
         // Being untitled let's our content take up the full frame.
         window.styleMask.remove(.titled)
 
+        // We dont' want the non-native fullscreen window to be resizable
+        // from the edges.
+        window.styleMask.remove(.resizable)
+
         // Focus window
         window.makeKeyAndOrderFront(nil)