fix(gtk): fix issue detecting preferred color scheme on older systems (…

…#4035)

On my system (pop-os 22.04 LTS) with system theme and seperate light and
dark themes, ghostty always defaults to light mode. Switches between
light and dark mode are properly handled.

In the logs this error is reported:

error(gtk): unable to get current color scheme:
GDBus.Error:org.freedesktop.DBus.Error.UnknownMethod: No such method
“ReadOne”

The spec notes that the other functions are "[Deprecated, use ReadOne
instead.](https://flatpak.github.io/xdg-desktop-portal/docs/doc-org.freedesktop.portal.Settings.html)"
so using ReadOne is cerainly the correct path.

I've managed to fix this on my system by checking for the error and
falling back to an implementation using the deprecated Read.

Discussion: #3704
Issue: #4038

src/apprt/gtk/App.zig

@@ -1400,7 +1400,15 @@ pub fn getColorScheme(self: *App) apprt.ColorScheme {
         null,
         &err,
     ) orelse {
-        if (err) |e| log.err("unable to get current color scheme: {s}", .{e.message});
+        if (err) |e| {
+            // If ReadOne is not yet implemented, fall back to deprecated "Read" method
+            // Error code: GDBus.Error:org.freedesktop.DBus.Error.UnknownMethod: No such method “ReadOne”
+            if (e.code == 19) {
+                return self.getColorSchemeDeprecated();
+            }
+            // Otherwise, log the error and return .light
+            log.err("unable to get current color scheme: {s}", .{e.message});
+        }
         return .light;
     };
     defer c.g_variant_unref(value);
@@ -1417,6 +1425,49 @@ pub fn getColorScheme(self: *App) apprt.ColorScheme {
     return .light;
 }
 
+/// Call the deprecated D-Bus "Read" method to determine the current color scheme. If
+/// there is any error at any point we'll log the error and return "light"
+fn getColorSchemeDeprecated(self: *App) apprt.ColorScheme {
+    const dbus_connection = c.g_application_get_dbus_connection(@ptrCast(self.app));
+    var err: ?*c.GError = null;
+    defer if (err) |e| c.g_error_free(e);
+
+    const value = c.g_dbus_connection_call_sync(
+        dbus_connection,
+        "org.freedesktop.portal.Desktop",
+        "/org/freedesktop/portal/desktop",
+        "org.freedesktop.portal.Settings",
+        "Read",
+        c.g_variant_new("(ss)", "org.freedesktop.appearance", "color-scheme"),
+        c.G_VARIANT_TYPE("(v)"),
+        c.G_DBUS_CALL_FLAGS_NONE,
+        -1,
+        null,
+        &err,
+    ) orelse {
+        if (err) |e| log.err("Read method failed: {s}", .{e.message});
+        return .light;
+    };
+    defer c.g_variant_unref(value);
+
+    if (c.g_variant_is_of_type(value, c.G_VARIANT_TYPE("(v)")) == 1) {
+        var inner: ?*c.GVariant = null;
+        c.g_variant_get(value, "(v)", &inner);
+        defer if (inner) |i| c.g_variant_unref(i);
+
+        if (inner) |i| {
+            const child = c.g_variant_get_child_value(i, 0) orelse {
+                return .light;
+            };
+            defer c.g_variant_unref(child);
+
+            const val = c.g_variant_get_uint32(child);
+            return if (val == 1) .dark else .light;
+        }
+    }
+    return .light;
+}
+
 /// This will be called by D-Bus when the style changes between light & dark.
 fn gtkNotifyColorScheme(
     _: ?*c.GDBusConnection,