Fix crash when closing the macos app from full screen (#2567)

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## Summary
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importantly what they aim to achieve -->
Strong reference was causing a crash when closing macos app from full
screen.

## Screenshots
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screenshots in light and dark mode. -->

## Link to pull request in Documentation repository
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corresponding pull request in the Companion App Documentation repository
(https://github.com/home-assistant/companion.home-assistant). Please add
the number of this pull request after the "#" -->
Documentation: home-assistant/companion.home-assistant#

## Any other notes
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Sources/App/Scenes/WebViewSceneDelegate.swift

@@ -71,6 +71,12 @@ final class WebViewSceneDelegate: NSObject, UIWindowSceneDelegate {
         #endif
     }
 
+    func sceneWillResignActive(_ scene: UIScene) {
+        #if targetEnvironment(macCatalyst)
+        WindowScenesManager.shared.sceneWillResignActive(scene)
+        #endif
+    }
+
     func sceneDidDisconnect(_ scene: UIScene) {
         windowController = nil
         window = nil

Sources/App/Scenes/WindowScenesManager.swift

@@ -12,13 +12,24 @@ final class WindowScenesManager {
         startObservingScene(scene)
     }
 
+    func sceneWillResignActive(_ scene: UIScene) {
+        stopObservingScene(scene)
+    }
+
+    func didDiscardScene(_ scene: UIScene) {
+        stopObservingScene(scene)
+    }
+
     private func startObservingScene(_ scene: UIWindowScene) {
         let observer = WindowSizeObserver(windowScene: scene)
         windowSizeObservers.append(observer)
     }
 
-    func didDiscardScene(_ scene: UIScene) {
-        windowSizeObservers.removeAll(where: { $0.observedScene == scene })
+    private func stopObservingScene(_ scene: UIScene) {
+        windowSizeObservers.first { observer in
+            observer.observedScene == scene
+        }?.stopObserving()
+        windowSizeObservers.removeAll { $0.observedScene == scene }
     }
 
     private func sceneFrameIsValid(_ sceneFrame: CGRect, screenSize: CGSize) -> Bool {

Sources/App/Scenes/WindowSizeObserver.swift

@@ -1,4 +1,5 @@
 import Foundation
+import Shared
 import UIKit
 
 final class WindowSizeObserver: NSObject {
@@ -8,9 +9,9 @@ final class WindowSizeObserver: NSObject {
     init(windowScene: UIWindowScene) {
         self.observedScene = windowScene
         super.init()
-        #if targetEnvironment(macCatalyst)
+
+        guard Current.isCatalyst else { return }
         startObserving()
-        #endif
     }
 
     private func startObserving() {
@@ -23,6 +24,11 @@ final class WindowSizeObserver: NSObject {
         }
         #endif
     }
+
+    public func stopObserving() {
+        observation?.invalidate()
+        observation = nil
+    }
 }
 
 enum ScenesWindowSizeConfig {