Fixes #12000: When viewport is set to camera and switched to SizedFul… (#12861)

# Objective

- When viewport is set to the same size as the window on creation, when
adjusting to SizedFullscreen, the window may be smaller than the
viewport for a moment, which caused the arguments to be invalid and
panic.
- Fixes #12000.

## Solution

- The fix consists of matching the size of the viewport to the lower
size of the window ( if the x value of the window is lower, I update
only the x value of the viewport, same for the y value). Also added a
test to show that it does not panic anymore.

---

crates/bevy_render/src/camera/camera.rs

@@ -755,6 +755,20 @@ pub fn camera_system<T: CameraProjection + Component>(
                         }
                     }
                 }
+                // This check is needed because when changing WindowMode to SizedFullscreen, the viewport may have invalid
+                // arguments due to a sudden change on the window size to a lower value.
+                // If the size of the window is lower, the viewport will match that lower value.
+                if let Some(viewport) = &mut camera.viewport {
+                    let target_info = &new_computed_target_info;
+                    if let Some(target) = target_info {
+                        if viewport.physical_size.x > target.physical_size.x {
+                            viewport.physical_size.x = target.physical_size.x;
+                        }
+                        if viewport.physical_size.y > target.physical_size.y {
+                            viewport.physical_size.y = target.physical_size.y;
+                        }
+                    }
+                }
                 camera.computed.target_info = new_computed_target_info;
                 if let Some(size) = camera.logical_viewport_size() {
                     camera_projection.update(size.x, size.y);

tests/window/change_window_mode.rs

@@ -0,0 +1,50 @@
+//! a test that confirms that 'bevy' does not panic while changing from Windowed to SizedFullscreen when viewport is set
+
+use bevy::
+{   prelude::*,
+    render::camera::Viewport,
+    window::WindowMode,
+};
+
+//Having a viewport set to the same size as a window used to cause panic on some occasions when switching to SizedFullscreen
+const WINDOW_WIDTH : f32 = 1366.0;
+const WINDOW_HEIGHT: f32 = 768.0;
+
+fn main()
+{   //Specify Window Size.
+    let window = Window { resolution: ( WINDOW_WIDTH, WINDOW_HEIGHT ).into(), ..default() };
+    let primary_window = Some ( window );
+
+    App::new()
+        .add_plugins( DefaultPlugins.set( WindowPlugin { primary_window, ..default() } ) )
+        .add_systems( Startup, startup )
+        .add_systems( Update, toggle_window_mode )
+        .run();
+}
+
+fn startup( mut cmds: Commands )
+{   //Match viewport to Window size.
+    let physical_position = UVec2::new( 0, 0 );
+    let physical_size = Vec2::new( WINDOW_WIDTH, WINDOW_HEIGHT ).as_uvec2();
+    let viewport = Some ( Viewport { physical_position, physical_size, ..default() } );
+
+    cmds.spawn( Camera2dBundle::default() ).insert( Camera { viewport, ..default() } );
+}
+
+fn toggle_window_mode
+(   mut qry_window: Query<&mut Window>,
+)
+{   let Ok( mut window ) = qry_window.get_single_mut() else { return };
+
+    window.mode = match window.mode {
+        WindowMode::Windowed => {
+            //it takes a while for the window to change from windowed to sizedfullscreen and back
+            std::thread::sleep(std::time::Duration::from_secs(4));
+            WindowMode::SizedFullscreen
+        },
+        _  => {
+            std::thread::sleep(std::time::Duration::from_secs(4));
+            WindowMode::Windowed
+        },
+    };
+}