macOS: React to changes in effective appearance of the NSApplication not the content view for each window

This is nicer now every OS Windows doesnt cause a notification. Also
fixes #9034 which was caused by us setting an explicit appearance on the
window when the titlebar is set to a specific color thereby preventing
the views in the window from getting appearance change notifications.

docs/changelog.rst

@@ -147,6 +147,10 @@ Detailed list of changes
 - macOS: Workaround for bug in macOS Tahoe that caused closed OS Windows to
   remain as invisible rectangles that intercept mouse events (:iss:`8952`)
 
+- macOS: Fix a regression in the previous release that broke automatic
+  switching of dark/light mode when setting :opt:`macos_titlebar_color` to a
+  arbitrary color (:iss:`9034`)
+
 - goto_session: Add ``--sort-by=alphabetical`` to have the interactive session
   picker list the sessions in a fixed order rather than by most recent
   (:disc:`9033`)

glfw/cocoa_init.m

@@ -297,6 +297,7 @@ static NSDictionary<NSString*,NSNumber*> *global_shortcuts = nil;
 // Delegate for application related notifications {{{
 
 @interface GLFWApplicationDelegate : NSObject <NSApplicationDelegate>
+    - (void)handleAppearanceChange;
 @end
 
 @implementation GLFWApplicationDelegate
@@ -415,20 +416,73 @@ static GLFWapplicationwillfinishlaunchingfun finish_launching_callback = NULL;
     }
 }
 
+static void *AppearanceObservationContext = &AppearanceObservationContext;
+
 - (void)applicationDidFinishLaunching:(NSNotification *)notification
 {
-    if (finish_launching_callback) finish_launching_callback(true);
     (void)notification;
+    [[NSApplication sharedApplication] addObserver:self
+        forKeyPath:@"effectiveAppearance" options:NSKeyValueObservingOptionNew | NSKeyValueObservingOptionInitial
+        context:AppearanceObservationContext];
+
+    if (finish_launching_callback) finish_launching_callback(true);
     [NSApp stop:nil];
 
     CGDisplayRegisterReconfigurationCallback(display_reconfigured, NULL);
     _glfwCocoaPostEmptyEvent();
 }
 
+GLFWAPI GLFWColorScheme glfwGetCurrentSystemColorTheme(bool query_if_unintialized) {
+    (void)query_if_unintialized;
+    int theme_type = GLFW_COLOR_SCHEME_NO_PREFERENCE;
+    NSAppearance *changedAppearance = NSApp.effectiveAppearance;
+    NSAppearanceName newAppearance = [changedAppearance bestMatchFromAppearancesWithNames:@[NSAppearanceNameAqua, NSAppearanceNameDarkAqua]];
+    if([newAppearance isEqualToString:NSAppearanceNameDarkAqua]){
+        theme_type = GLFW_COLOR_SCHEME_DARK;
+    } else {
+        theme_type = GLFW_COLOR_SCHEME_LIGHT;
+    }
+    return theme_type;
+}
+
+- (void)observeValueForKeyPath:(NSString *)keyPath
+                      ofObject:(id)object
+                        change:(NSDictionary<NSKeyValueChangeKey, id> *)change
+                       context:(void *)context {
+                           if (context == AppearanceObservationContext) {
+        if ([keyPath isEqualToString:@"effectiveAppearance"]) {
+            // The initial call (from NSKeyValueObservingOptionInitial) might happen on a background thread.
+            // Dispatch to the main thread to be safe, especially if updating UI.
+            dispatch_async(dispatch_get_main_queue(), ^{
+                [self handleAppearanceChange];
+            });
+        }
+    } else {
+        // If the context doesn't match, pass the notification to the superclass.
+        [super observeValueForKeyPath:keyPath ofObject:object change:change context:context];
+    }
+}
+
+- (void)handleAppearanceChange {
+    static GLFWColorScheme previously_reported_appearance = GLFW_COLOR_SCHEME_NO_PREFERENCE;
+    GLFWColorScheme new_appearance = glfwGetCurrentSystemColorTheme(true);
+    if (new_appearance != previously_reported_appearance) {
+        previously_reported_appearance = new_appearance;
+        _glfwInputColorScheme(new_appearance, false);
+    }
+}
+
 - (void)applicationWillTerminate:(NSNotification *)aNotification
 {
     (void)aNotification;
     CGDisplayRemoveReconfigurationCallback(display_reconfigured, NULL);
+    @try {
+        [[NSApplication sharedApplication] removeObserver:self
+                                               forKeyPath:@"effectiveAppearance"
+                                                  context:AppearanceObservationContext];
+    } @catch (NSException * __unused exception) {
+        // Ignore exceptions, which can happen if the observer was never added.
+    }
 }
 
 - (void)applicationDidHide:(NSNotification *)notification

glfw/cocoa_window.m

@@ -981,16 +981,6 @@ static const NSRange kEmptyRange = { NSNotFound, 0 };
     updateCursorImage(window);
 }
 
-- (void)viewDidChangeEffectiveAppearance
-{
-    static GLFWColorScheme appearance = GLFW_COLOR_SCHEME_NO_PREFERENCE;
-    GLFWColorScheme new_appearance = glfwGetCurrentSystemColorTheme(true);
-    if (new_appearance != appearance) {
-        appearance = new_appearance;
-        _glfwInputColorScheme(appearance, false);
-    }
-}
-
 - (void)viewDidChangeBackingProperties
 {
     if (!window) return;
@@ -3434,20 +3424,6 @@ GLFWAPI void glfwCocoaSetWindowChrome(GLFWwindow *w, unsigned int color, bool us
     [window->ns.object makeFirstResponder:window->ns.view];
 }}
 
-GLFWAPI GLFWColorScheme glfwGetCurrentSystemColorTheme(bool query_if_unintialized) {
-    (void)query_if_unintialized;
-    int theme_type = 0;
-    NSAppearance *changedAppearance = NSApp.effectiveAppearance;
-    NSAppearanceName newAppearance = [changedAppearance bestMatchFromAppearancesWithNames:@[NSAppearanceNameAqua, NSAppearanceNameDarkAqua]];
-    if([newAppearance isEqualToString:NSAppearanceNameDarkAqua]){
-        theme_type = 1;
-    } else {
-        theme_type = 2;
-    }
-    return theme_type;
-}
-
-
 GLFWAPI uint32_t
 glfwGetCocoaKeyEquivalent(uint32_t glfw_key, int glfw_mods, int *cocoa_mods) {
     *cocoa_mods = 0;