macos: new tab implementation

macos/Sources/GhosttyApp.swift

@@ -23,6 +23,20 @@ struct GhosttyApp: App {
                 TerminalView(app: ghostty.app!)
                     .modifier(WindowObservationModifier())
             }
+        }.commands {
+            CommandGroup(after: .newItem) {
+                Button("New Tab", action: newTab).keyboardShortcut("t", modifiers: [.command])
+            }
+        }
+    }
+    
+    // Create a new tab in the currently active window
+    func newTab() {
+        guard let currentWindow = NSApp.keyWindow else { return }
+        guard let windowController = currentWindow.windowController else { return }
+        windowController.newWindowForTab(nil)
+        if let newWindow = NSApp.keyWindow, currentWindow != newWindow {
+            currentWindow.addTabbedWindow(newWindow, ordered: .above)
         }
     }
 }

macos/Sources/TerminalView.swift

@@ -5,6 +5,7 @@ struct TerminalView: View {
     let app: ghostty_app_t
     @FocusState private var surfaceFocus: Bool
     @Environment(\.isKeyWindow) private var isKeyWindow: Bool
+    @Environment(\.openWindow) private var openWindow
     @State private var title: String = "Ghostty"
     
     // This is true if the terminal is considered "focused". The terminal is focused if

src/App.zig

@@ -219,6 +219,8 @@ fn newTab(self: *App, msg: Message.NewWindow) !void {
         return;
     }
 
+    // In embedded mode, it is up to the embedder to implement tabbing
+    // on their own.
     if (comptime build_config.artifact != .exe) {
         log.warn("tabbing is not supported in embedded mode", .{});
         return;