Fix: First frame of Context::create_viewport_sync was always skiped because we didn't have a window to draw on!

Changes: Now Context::create_viewport_sync returns T, before was Option<T>

crates/egui/src/containers/window.rs

@@ -396,7 +396,7 @@ impl<'open> Window<'open> {
                         .with_max_inner_size(Some((max_size.x as u32, max_size.y as u32)));
                 }
 
-                return ctx.create_viewport_sync(window_builder, move |ctx| {
+                return Some(ctx.create_viewport_sync(window_builder, move |ctx| {
                     let mut op = is_open;
                     let open = if show_close_button {
                         Some(&mut op)
@@ -625,7 +625,7 @@ impl<'open> Window<'open> {
                         inner: content_inner,
                         response: full_response,
                     }
-                });
+                }));
             }
         }
         let frame = frame.unwrap_or_else(|| Frame::window(&ctx.style()));

crates/egui/src/context.rs

@@ -2318,7 +2318,7 @@ impl Context {
         &self,
         viewport_builder: ViewportBuilder,
         func: impl FnOnce(&Context) -> T,
-    ) -> Option<T> {
+    ) -> T {
         if !self.force_embedding() {
             let mut viewport_id = ViewportId::MAIN;
             let mut parent_viewport_id = ViewportId::MAIN;
@@ -2355,9 +2355,9 @@ impl Context {
                 );
             }
 
-            out
+            out.expect("egui backend is implemented incorrectly! Context::set_render_sync_callback")
         } else {
-            Some(func(self))
+            func(self)
         }
     }
 }