apprt/gtk: add menu to new tab button to create splits (#7137)

Closes: https://github.com/ghostty-org/ghostty/discussions/6828

src/apprt/gtk/Window.zig

@@ -25,6 +25,7 @@ const input = @import("../../input.zig");
 const CoreSurface = @import("../../Surface.zig");
 
 const App = @import("App.zig");
+const Builder = @import("Builder.zig");
 const Color = configpkg.Config.Color;
 const Surface = @import("Surface.zig");
 const Menu = @import("menu.zig").Menu;
@@ -242,12 +243,19 @@ pub fn init(self: *Window, app: *App) !void {
     }
 
     {
-        const btn = gtk.Button.newFromIconName("tab-new-symbolic");
+        const btn = adw.SplitButton.new();
+        btn.setIconName("tab-new-symbolic");
         btn.as(gtk.Widget).setTooltipText(i18n._("New Tab"));
-        _ = gtk.Button.signals.clicked.connect(
+        btn.setDropdownTooltip(i18n._("New Split"));
+
+        var builder = Builder.init("menu-headerbar-split_menu", 1, 0, .blp);
+        defer builder.deinit();
+        btn.setMenuModel(builder.getObject(gio.MenuModel, "menu"));
+
+        _ = adw.SplitButton.signals.clicked.connect(
             btn,
             *Window,
-            gtkTabNewClick,
+            adwNewTabClick,
             self,
             .{},
         );
@@ -824,6 +832,11 @@ fn gtkTabNewClick(_: *gtk.Button, self: *Window) callconv(.c) void {
     self.performBindingAction(.{ .new_tab = {} });
 }
 
+/// Create a new surface (tab or split).
+fn adwNewTabClick(_: *adw.SplitButton, self: *Window) callconv(.c) void {
+    self.performBindingAction(.{ .new_tab = {} });
+}
+
 /// Create a new tab from the AdwTabOverview. We can't copy gtkTabNewClick
 /// because we need to return an AdwTabPage from this function.
 fn gtkNewTabFromOverview(_: *adw.TabOverview, self: *Window) callconv(.c) *adw.TabPage {

src/apprt/gtk/gresource.zig

@@ -75,6 +75,7 @@ pub const VersionedBlueprint = struct {
 pub const blueprint_files = [_]VersionedBlueprint{
     .{ .major = 1, .minor = 5, .name = "prompt-title-dialog" },
     .{ .major = 1, .minor = 5, .name = "config-errors-dialog" },
+    .{ .major = 1, .minor = 0, .name = "menu-headerbar-split_menu" },
     .{ .major = 1, .minor = 0, .name = "menu-surface-context_menu" },
     .{ .major = 1, .minor = 0, .name = "menu-window-titlebar_menu" },
     .{ .major = 1, .minor = 5, .name = "ccw-osc-52-read" },

src/apprt/gtk/ui/1.0/menu-headerbar-split_menu.blp

@@ -0,0 +1,25 @@
+using Gtk 4.0;
+
+menu menu {
+  section {
+    item {
+      label: _("Split Up");
+      action: "win.split-up";
+    }
+
+    item {
+      label: _("Split Down");
+      action: "win.split-down";
+    }
+
+    item {
+      label: _("Split Left");
+      action: "win.split-left";
+    }
+
+    item {
+      label: _("Split Right");
+      action: "win.split-right";
+    }
+  }
+}