Actually allow windows to be upcasted

src/platform_impl/android/mod.rs

@@ -985,6 +985,10 @@ impl CoreWindow for Window {
     }
 
     fn reset_dead_keys(&self) {}
+
+    fn as_surface(&self) -> &dyn CoreSurface {
+        self
+    }
 }
 
 #[derive(Default, Clone, Debug)]

src/platform_impl/apple/appkit/window.rs

@@ -338,6 +338,10 @@ impl CoreWindow for Window {
     fn show_window_menu(&self, position: Position) {
         self.maybe_wait_on_main(|delegate| delegate.show_window_menu(position))
     }
+
+    fn as_surface(&self) -> &dyn CoreSurface {
+        self
+    }
 }
 
 declare_class!(

src/platform_impl/apple/uikit/window.rs

@@ -830,6 +830,10 @@ impl CoreWindow for Window {
     fn show_window_menu(&self, position: Position) {
         self.maybe_wait_on_main(|delegate| delegate.show_window_menu(position))
     }
+
+    fn as_surface(&self) -> &dyn CoreSurface {
+        self
+    }
 }
 
 // WindowExtIOS

src/platform_impl/linux/wayland/window/mod.rs

@@ -664,6 +664,10 @@ impl CoreWindow for Window {
         let position = position.to_logical(scale_factor);
         self.window_state.lock().unwrap().show_window_menu(position);
     }
+
+    fn as_surface(&self) -> &dyn CoreSurface {
+        self
+    }
 }
 
 /// The request from the window to the event loop.

src/platform_impl/linux/x11/window.rs

@@ -309,6 +309,10 @@ impl CoreWindow for Window {
     fn show_window_menu(&self, position: Position) {
         self.0.show_window_menu(position);
     }
+
+    fn as_surface(&self) -> &dyn CoreSurface {
+        self
+    }
 }
 
 #[cfg(feature = "rwh_06")]

src/platform_impl/orbital/window.rs

@@ -464,6 +464,10 @@ impl CoreWindow for Window {
     fn set_theme(&self, _theme: Option<window::Theme>) {}
 
     fn set_content_protected(&self, _protected: bool) {}
+
+    fn as_surface(&self) -> &dyn CoreSurface {
+        self
+    }
 }
 
 #[cfg(feature = "rwh_06")]

src/platform_impl/web/window.rs

@@ -390,6 +390,10 @@ impl RootWindow for Window {
     }
 
     fn show_window_menu(&self, _: Position) {}
+
+    fn as_surface(&self) -> &dyn CoreSurface {
+        self
+    }
 }
 
 #[cfg(feature = "rwh_06")]

src/platform_impl/windows/window.rs

@@ -1068,6 +1068,10 @@ impl CoreWindow for Window {
             );
         }
     }
+
+    fn as_surface(&self) -> &dyn CoreSurface {
+        self
+    }
 }
 
 pub(super) struct InitData<'a> {

src/window.rs

@@ -1310,9 +1310,7 @@ pub trait Window: Surface {
     fn show_window_menu(&self, position: Position);
 
     /// Upcasts this `Window` to a [`Surface`]. 
-    fn as_surface(&self) -> &dyn Surface where Self: Sized {
-        self
-    }
+    fn as_surface(&self) -> &dyn Surface;
 }
 
 impl dyn Window {