macOS: fix theme reloading

### Background
After #9344, the Ghostty theme won't change after switching systems', and reverting #9344 will bring back the issue it fixed.

The reason these two issues are related is because the scheme change is based on changes of `effectiveAppearance`, which is also affected by setting the window's `appearance` or changing `NSAppearance.currentDrawing()`.

### Changes
Instead of observing `effectiveAppearance`, we now explicitly update the color scheme of surfaces, so that we can control when it happens to avoid callback loops and redundant updates.

### Regression Tests

- [x] #8282
- [x] Reloading with `window-theme = light` should update Ghostty with the default dark theme with a dark window theme (break before [#83104ff](83104ff27a))
- [x] `window-theme = light \n macos-titlebar-style = native` should update Ghostty with the default dark theme with a light window theme
- [x] Reloading from the default config to `theme=light:3024 Day,dark:3024 Night \n window-theme = light`, should update Ghostty with the theme `3024 Day` with a light window theme (break on [#d39cc6d](d39cc6d478))
- [x] Using `theme=light:3024 Day,dark:3024 Night`; Switching the system's appearance should change Ghostty's appearance (break on [#d39cc6d](d39cc6d478))
- [x] Reloading from `theme=light:3024 Day,dark:3024 Night` with a light window theme to the default config, should update Ghostty with the default dark theme with a dark window theme
- [x] Reloading from the default config to `theme=light:3024 Day,dark:3024 Night \n window-theme=dark`, should update Ghostty with the theme `3024 Night` with a dark window theme
- [x] Reloading from `theme=light:3024 Day,dark:3024 Night \n window-theme=dark` to `theme=light:3024 Day,dark:3024 Night` with light system appearance, should update Ghostty from dark to light
- [x] Reload with quick terminal open

# Conflicts:
#	macos/Sources/Features/Terminal/BaseTerminalController.swift

macos/Sources/Features/QuickTerminal/QuickTerminalController.swift

@@ -566,6 +566,7 @@ class QuickTerminalController: BaseTerminalController {
     private func syncAppearance() {
         guard let window else { return }
 
+        defer { updateColorSchemeForSurfaceTree() }
         // Change the collection behavior of the window depending on the configuration.
         window.collectionBehavior = derivedConfig.quickTerminalSpaceBehavior.collectionBehavior
 

macos/Sources/Features/Terminal/BaseTerminalController.swift

@@ -72,6 +72,9 @@ class BaseTerminalController: NSWindowController,
     /// The previous frame information from the window
     private var savedFrame: SavedFrame? = nil
 
+    /// Cache previously applied appearance to avoid unnecessary updates
+    private var appliedColorScheme: ghostty_color_scheme_e?
+
     /// The configuration derived from the Ghostty config so we don't need to rely on references.
     private var derivedConfig: DerivedConfig
 
@@ -1163,4 +1166,35 @@ extension BaseTerminalController: NSMenuItemValidation {
             return true
         }
     }
+	
+    // MARK: - Surface Color Scheme
+
+    /// Update the surface tree's color scheme only when it actually changes.
+    ///
+    /// Calling ``ghostty_surface_set_color_scheme`` triggers
+    /// ``syncAppearance(_:)`` via notification,
+    /// so we avoid redundant calls.
+    func updateColorSchemeForSurfaceTree() {
+        /// Derive the target scheme from `window-theme` or system appearance.
+        /// We set the scheme on surfaces so they pick the correct theme
+        /// and let ``syncAppearance(_:)`` update the window accordingly.
+        ///
+        /// Using App's effectiveAppearance here to prevent incorrect updates.
+        let themeAppearance = NSApplication.shared.effectiveAppearance
+        let scheme: ghostty_color_scheme_e
+        if themeAppearance.isDark {
+            scheme = GHOSTTY_COLOR_SCHEME_DARK
+        } else {
+            scheme = GHOSTTY_COLOR_SCHEME_LIGHT
+        }
+        guard scheme != appliedColorScheme else {
+            return
+        }
+        for surfaceView in surfaceTree {
+            if let surface = surfaceView.surface {
+                ghostty_surface_set_color_scheme(surface, scheme)
+            }
+        }
+        appliedColorScheme = scheme
+    }
 }

macos/Sources/Features/Terminal/TerminalController.swift

@@ -425,15 +425,9 @@ class TerminalController: BaseTerminalController, TabGroupCloseCoordinator.Contr
 
             return
         }
-
-        // This is a surface-level config update. If we have the surface, we
-        // update our appearance based on it.
-        guard let surfaceView = notification.object as? Ghostty.SurfaceView else { return }
-        guard surfaceTree.contains(surfaceView) else { return }
-
-        // We can't use surfaceView.derivedConfig because it may not be updated
-        // yet since it also responds to notifications.
-        syncAppearance(.init(config))
+        /// Surface-level config will be updated in
+        /// ``Ghostty/Ghostty/SurfaceView/derivedConfig`` then
+        /// ``TerminalController/focusedSurfaceDidChange(to:)``
     }
 
     /// Update the accessory view of each tab according to the keyboard

macos/Sources/Features/Terminal/Window Styles/TerminalWindow.swift

@@ -419,6 +419,7 @@ class TerminalWindow: NSWindow {
         // have no effect if the window is not visible. Ultimately, we'll have this called
         // at some point when a surface becomes focused.
         guard isVisible else { return }
+        defer { updateColorSchemeForSurfaceTree() }
 
         // Basic properties
         appearance = surfaceConfig.windowAppearance
@@ -481,6 +482,10 @@ class TerminalWindow: NSWindow {
         return derivedConfig.backgroundColor.withAlphaComponent(alpha)
     }
 
+    func updateColorSchemeForSurfaceTree() {
+        terminalController?.updateColorSchemeForSurfaceTree()
+    }
+
     private func setInitialWindowPosition(x: Int16?, y: Int16?) {
         // If we don't have an X/Y then we try to use the previously saved window pos.
         guard let x, let y else {

macos/Sources/Ghostty/SurfaceView_AppKit.swift

@@ -369,26 +369,6 @@ extension Ghostty {
             // Setup our tracking area so we get mouse moved events
             updateTrackingAreas()
 
-            // Observe our appearance so we can report the correct value to libghostty.
-            // This is the best way I know of to get appearance change notifications.
-            self.appearanceObserver = observe(\.effectiveAppearance, options: [.new, .initial]) { view, change in
-                guard let appearance = change.newValue else { return }
-                guard let surface = view.surface else { return }
-                let scheme: ghostty_color_scheme_e
-                switch (appearance.name) {
-                case .aqua, .vibrantLight:
-                    scheme = GHOSTTY_COLOR_SCHEME_LIGHT
-
-                case .darkAqua, .vibrantDark:
-                    scheme = GHOSTTY_COLOR_SCHEME_DARK
-
-                default:
-                    return
-                }
-
-                ghostty_surface_set_color_scheme(surface, scheme)
-            }
-
             // The UTTypes that can be dragged onto this view.
             registerForDraggedTypes(Array(Self.dropTypes))
         }