macOS: Decouple screen property updates from application delegate

Change-Id: I489c37131bf715d45f147964de4a8cd8c02adbcb
Fixes: QTBUG-72966
Reviewed-by: Timur Pocheptsov <timur.pocheptsov@qt.io>
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@qt.io>

src/plugins/platforms/cocoa/qcocoaapplicationdelegate.mm

@@ -115,22 +115,10 @@ QT_USE_NAMESPACE
     self = [super init];
     if (self) {
         inLaunch = true;
-        [[NSNotificationCenter defaultCenter]
-                addObserver:self
-                   selector:@selector(updateScreens:)
-                       name:NSApplicationDidChangeScreenParametersNotification
-                     object:NSApp];
     }
     return self;
 }
 
-- (void)updateScreens:(NSNotification *)notification
-{
-    Q_UNUSED(notification);
-    if (QCocoaIntegration *ci = QCocoaIntegration::instance())
-        ci->updateScreens();
-}
-
 - (void)dealloc
 {
     [_dockMenu release];

src/plugins/platforms/cocoa/qcocoaintegration.h

@@ -144,6 +144,7 @@ private:
 #endif
     QScopedPointer<QPlatformTheme> mPlatformTheme;
     QList<QCocoaScreen *> mScreens;
+    QMacScopedObserver m_screensObserver;
 #ifndef QT_NO_CLIPBOARD
     QCocoaClipboard  *mCocoaClipboard;
 #endif

src/plugins/platforms/cocoa/qcocoaintegration.mm

@@ -206,6 +206,9 @@ QCocoaIntegration::QCocoaIntegration(const QStringList &paramList)
     // by explicitly setting the presentation option to the magic 'default value',
     // which will resolve to an actual value and result in screen invalidation.
     cocoaApplication.presentationOptions = NSApplicationPresentationDefault;
+
+    m_screensObserver = QMacScopedObserver([NSApplication sharedApplication],
+        NSApplicationDidChangeScreenParametersNotification, [&]() { updateScreens(); });
     updateScreens();
 
     QMacInternalPasteboardMime::initializeMimeTypes();