Fixes bevyengine#12000: When viewport is set to camera and switched t…

…o SizedFullscreen, panic may occur

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.
The fix consists of matching the size of the viewport to the size of the window when it is smaller.
Also added a test to show that it does not panic.

crates/bevy_render/src/camera/camera.rs

@@ -755,6 +755,17 @@ pub fn camera_system<T: CameraProjection + Component>(
                         }
                     }
                 }
+                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
+        },
+    };
+}