Avoid a "RefCell already borrowed" error with WINIT_WINDOWS (#21338)

# Objective

Fixes #21319.

## Solution

In the "about_to_wait" state of Bevy's winit event loop, we borrow
WINIT_WINDOWS only to check the conditions for a redraw, and perform the
call after that check is concluded.

## Testing

Tested by confirming that the RefCell error no longer occurs in my
application. In my application, the error occurred when the
`bevy-egui-inspector` crate was used.

I do not know whether the original reporter of #21319 was also using
third-party Bevy plugins that might have caused the issue. It would be
good if they could check whether this change fixes it for them.

Tested on Windows 11, where the bug is known to occur. The code that was
changed is only included in Windows builds so there is little need to
test on other platforms.

---------

Co-authored-by: Alice Cecile <[email protected]>
Co-authored-by: Dimitrios Loukadakis <[email protected]>

Author: 3 people

crates/bevy_winit/src/state.rs

@@ -466,24 +466,23 @@ impl<M: Message> ApplicationHandler<M> for WinitAppRunnerState<M> {
         // invisible window creation. https://github.com/bevyengine/bevy/issues/18027
         #[cfg(target_os = "windows")]
         {
-            WINIT_WINDOWS.with_borrow(|winit_windows| {
-                let headless = winit_windows.windows.is_empty();
-                let exiting = self.app_exit.is_some();
-                let reactive = matches!(self.update_mode, UpdateMode::Reactive { .. });
-                let all_invisible = winit_windows
-                    .windows
-                    .iter()
-                    .all(|(_, w)| !w.is_visible().unwrap_or(false));
-                if !exiting
-                    && (self.startup_forced_updates > 0
-                        || headless
-                        || all_invisible
-                        || reactive
-                        || self.window_event_received)
-                {
-                    self.redraw_requested(event_loop);
-                }
-            });
+            fn headless_or_all_invisible() -> bool {
+                WINIT_WINDOWS.with_borrow(|winit_windows| {
+                    winit_windows
+                        .windows
+                        .iter()
+                        .all(|(_, w)| !w.is_visible().unwrap_or(false))
+                })
+            }
+
+            if !self.app_exit.is_some()
+                && (self.startup_forced_updates > 0
+                    || matches!(self.update_mode, UpdateMode::Reactive { .. })
+                    || self.window_event_received
+                    || headless_or_all_invisible())
+            {
+                self.redraw_requested(event_loop);
+            }
         }
     }
 
@@ -1023,10 +1022,10 @@ mod tests {
         app.add_systems(
             Update,
             move |mut window: Single<(Entity, &mut Window)>,
-             mut window_backend_scale_factor_changed: MessageWriter<
-                WindowBackendScaleFactorChanged,
-            >,
-             mut window_scale_factor_changed: MessageWriter<WindowScaleFactorChanged>| {
+                  mut window_backend_scale_factor_changed: MessageWriter<
+                      WindowBackendScaleFactorChanged,
+                  >,
+                  mut window_scale_factor_changed: MessageWriter<WindowScaleFactorChanged>| {
                 react_to_scale_factor_change(
                     window.0,
                     &mut window.1,