apprt/gtk: gtk-single-instance defaults to true only if desktop env

Fixes #473

A new config value for `gtk-single-instance = desktop` is now valid.
This value uses GTK single instance mode only if it detects Ghostty was
launched from a desktop environment (i.e. clicking on the icon) through
the `.desktop` Freedesktop file. Otherwise, it disables single instance.

This enables Ghostty to work well with the CLI and accept all the
typical CLI flags. It will make Ghostty launch slower from the CLI but I
think this is an acceptable tradeoff.

src/apprt/gtk/App.zig

@@ -16,6 +16,7 @@ const builtin = @import("builtin");
 const glfw = @import("glfw");
 const configpkg = @import("../../config.zig");
 const input = @import("../../input.zig");
+const internal_os = @import("../../os/main.zig");
 const Config = configpkg.Config;
 const CoreApp = @import("../../App.zig");
 const CoreSurface = @import("../../Surface.zig");
@@ -72,10 +73,16 @@ pub fn init(core_app: *CoreApp, opts: Options) !App {
     const cursor_none = c.gdk_cursor_new_from_name("none", null);
     errdefer if (cursor_none) |cursor| c.g_object_unref(cursor);
 
+    const single_instance = switch (config.@"gtk-single-instance") {
+        .true => true,
+        .false => false,
+        .desktop => internal_os.launchedFromDesktop(),
+    };
+
     // Setup the flags for our application.
     const app_flags: c.GApplicationFlags = app_flags: {
         var flags: c.GApplicationFlags = c.G_APPLICATION_DEFAULT_FLAGS;
-        if (!config.@"gtk-single-instance") flags |= c.G_APPLICATION_NON_UNIQUE;
+        if (!single_instance) flags |= c.G_APPLICATION_NON_UNIQUE;
         break :app_flags flags;
     };
 
@@ -88,6 +95,10 @@ pub fn init(core_app: *CoreApp, opts: Options) !App {
     };
 
     // Create our GTK Application which encapsulates our process.
+    log.debug("creating GTK application id={s} single-instance={}", .{
+        app_id,
+        single_instance,
+    });
     const app = @as(?*c.GtkApplication, @ptrCast(c.gtk_application_new(
         app_id.ptr,
         app_flags,

src/config/Config.zig

@@ -406,14 +406,19 @@ keybind: Keybinds = .{},
 /// This does not work with GLFW builds.
 @"macos-option-as-alt": OptionAsAlt = .false,
 
-/// If true (default), then the Ghostty GTK application will run in
+/// If true, the Ghostty GTK application will run in single-instance mode:
-/// single-instance mode: each new `ghostty` process launched will result
+/// each new `ghostty` process launched will result in a new window if there
-/// in a new window, if there is already a running process.
+/// is already a running process.
 ///
 /// If false, each new ghostty process will launch a separate application.
 ///
-/// Debug builds of Ghostty have a separate single-instance ID.
+/// The default value is "desktop" which will default to "true" if Ghostty
-@"gtk-single-instance": bool = true,
+/// detects it was launched from the .desktop file such as an app launcher.
+/// If Ghostty is launched from the command line, it will default to "false".
+///
+/// Note that debug builds of Ghostty have a separate single-instance ID
+/// so you can test single instance without conflicting with release builds.
+@"gtk-single-instance": GtkSingleInstance = .desktop,
 
 /// If true (default), then the Ghostty GTK tabs will be "wide." Wide tabs
 /// are the new typical Gnome style where tabs fill their available space.
@@ -1856,3 +1861,10 @@ pub const WindowTheme = enum {
     light,
     dark,
 };
+
+/// See gtk-single-instance
+pub const GtkSingleInstance = enum {
+    desktop,
+    false,
+    true,
+};