renderer/metal: API to handle macOS display ID changing

include/ghostty.h

@@ -542,6 +542,10 @@ uintptr_t ghostty_surface_pwd(ghostty_surface_t, char*, uintptr_t);
 bool ghostty_surface_has_selection(ghostty_surface_t);
 uintptr_t ghostty_surface_selection(ghostty_surface_t, char*, uintptr_t);
 
+#ifdef __APPLE__
+void ghostty_surface_set_display_id(ghostty_surface_t, uint32_t);
+#endif
+
 ghostty_inspector_t ghostty_surface_inspector(ghostty_surface_t);
 void ghostty_inspector_free(ghostty_surface_t);
 void ghostty_inspector_set_focus(ghostty_inspector_t, bool);

macos/Sources/Ghostty/SurfaceView_AppKit.swift

@@ -112,6 +112,11 @@ extension Ghostty {
                 selector: #selector(onUpdateRendererHealth),
                 name: Ghostty.Notification.didUpdateRendererHealth,
                 object: self)
+            center.addObserver(
+                self,
+                selector: #selector(windowDidChangeScreen),
+                name: NSWindow.didChangeScreenNotification,
+                object: nil)
             
             // Setup our surface. This will also initialize all the terminal IO.
             let surface_cfg = baseConfig ?? SurfaceConfiguration()
@@ -322,6 +327,19 @@ extension Ghostty {
             healthy = health == GHOSTTY_RENDERER_HEALTH_OK
         }
         
+        @objc private func windowDidChangeScreen(notification: SwiftUI.Notification) {
+            guard let window = self.window else { return }
+            guard let object = notification.object as? NSWindow, window == object else { return }
+            guard let screen = window.screen else { return }
+            guard let surface = self.surface else { return }
+            
+            // When the window changes screens, we need to update libghostty with the screen
+            // ID. If vsync is enabled, this will be used with the CVDisplayLink to ensure
+            // the proper refresh rate is going.
+            let id = (screen.deviceDescription[NSDeviceDescriptionKey("NSScreenNumber")] as! NSNumber).uint32Value
+            ghostty_surface_set_display_id(surface, id)
+        }
+        
         // MARK: - NSView
         
         override func viewDidMoveToWindow() {

src/apprt/embedded.zig

@@ -1737,6 +1737,15 @@ pub const CAPI = struct {
 
     // Inspector Metal APIs are only available on Apple systems
     usingnamespace if (builtin.target.isDarwin()) struct {
+        export fn ghostty_surface_set_display_id(ptr: *Surface, display_id: u32) void {
+            const surface = &ptr.core_surface;
+            _ = surface.renderer_thread.mailbox.push(
+                .{ .macos_display_id = display_id },
+                .{ .forever = {} },
+            );
+            surface.renderer_thread.wakeup.notify() catch {};
+        }
+
         export fn ghostty_inspector_metal_init(ptr: *Inspector, device: objc.c.id) bool {
             return ptr.initMetal(objc.Object.fromId(device));
         }

src/renderer/Metal.zig

@@ -561,10 +561,14 @@ pub fn init(alloc: Allocator, options: renderer.Options) !Metal {
     var cells = try mtl_cell.Contents.init(alloc);
     errdefer cells.deinit(alloc);
 
-    const display_link: ?DisplayLink = switch (builtin.os.tag) {
+    const display_link: ?DisplayLink = null;
-        .macos => try macos.video.DisplayLink.createWithActiveCGDisplays(),
+    // Note(mitchellh): if/when we ever want to add vsync, we can use this
-        else => null,
+    // display link to trigger rendering. We don't need this if vsync is off
-    };
+    // because any change will trigger a redraw immediately.
+    // const display_link: ?DisplayLink = switch (builtin.os.tag) {
+    //     .macos => try macos.video.DisplayLink.createWithActiveCGDisplays(),
+    //     else => null,
+    // };
     errdefer if (display_link) |v| v.release();
 
     return Metal{
@@ -701,6 +705,16 @@ fn displayLinkCallback(
     };
 }
 
+/// Called when we get an updated display ID for our display link.
+pub fn setMacOSDisplayID(self: *Metal, id: u32) !void {
+    if (comptime DisplayLink == void) return;
+    const display_link = self.display_link orelse return;
+    log.info("updating display link display id={}", .{id});
+    display_link.setCurrentCGDisplay(id) catch |err| {
+        log.warn("error setting display link display id err={}", .{err});
+    };
+}
+
 /// True if our renderer has animations so that a higher frequency
 /// timer is used.
 pub fn hasAnimations(self: *const Metal) bool {

src/renderer/Thread.zig

@@ -375,6 +375,12 @@ fn drainMailbox(self: *Thread) !void {
             },
 
             .inspector => |v| self.flags.has_inspector = v,
+
+            .macos_display_id => |v| {
+                if (@hasDecl(renderer.Renderer, "setMacOSDisplayID")) {
+                    try self.renderer.setMacOSDisplayID(v);
+                }
+            },
         }
     }
 }

src/renderer/message.zig

@@ -69,6 +69,9 @@ pub const Message = union(enum) {
     /// Activate or deactivate the inspector.
     inspector: bool,
 
+    /// The macOS display ID has changed for the window.
+    macos_display_id: u32,
+
     /// Initialize a change_config message.
     pub fn initChangeConfig(alloc: Allocator, config: *const configpkg.Config) !Message {
         const thread_ptr = try alloc.create(renderer.Thread.DerivedConfig);