csd-background: Check for layer-shell support before choosing which

Gdk backends to allow.

We want to allow wayland *only* if it supports the layer-shell
protocol.

Author: mtwebster

debian/control

@@ -25,6 +25,7 @@ Build-Depends:
  libsystemd-dev [linux-any],
  libupower-glib-dev (>= 0.99.11),
  libwacom-dev (>= 0.4) [!s390x !hurd-any !kfreebsd-any],
+ libwayland-dev,
  libx11-dev,
  libxext-dev,
  libxi-dev,

meson.build

@@ -91,8 +91,10 @@ endif
 
 gtk_layer_shell_enabled = get_option('gtk_layer_shell')
 gtk_layer_shell = dependency('', required: false)
+wayland_client = dependency('', required: false)
 if gtk_layer_shell_enabled
     gtk_layer_shell = dependency('gtk-layer-shell-0', version: '>= 0.8', required: true)
+    wayland_client = dependency('wayland-client', required: true)
 endif
 
 cc = meson.get_compiler('c')

plugins/background/main.c

@@ -1,5 +1,8 @@
 #include "config.h"
 
+#include <glib.h>
+#include <gdk/gdk.h>
+
 #define NEW csd_background_manager_new
 #define START csd_background_manager_start
 #define STOP csd_background_manager_stop
@@ -14,9 +17,21 @@
 // Setting this to TRUE makes the plugin force GDK_SCALE=1
 #define FORCE_GDK_SCALE TRUE
 
-// Run as native Wayland client when gtk-layer-shell is available,
-// otherwise fall back to x11/xwayland
+// When gtk-layer-shell is available, check at runtime if the compositor
+// supports wlr-layer-shell. If not, fall back to X11/XWayland.
 #ifdef HAVE_GTK_LAYER_SHELL
+#include "wayland-utils.h"
+
+static void
+pre_gtk_init (void)
+{
+    if (!csd_check_layer_shell_support ()) {
+        g_message ("csd-background: wlr-layer-shell not supported, using X11 backend");
+        gdk_set_allowed_backends ("x11");
+    }
+}
+
+#define PRE_GTK_INIT pre_gtk_init
 #define FORCE_X11_BACKEND FALSE
 #else
 #define FORCE_X11_BACKEND TRUE

plugins/common/daemon-skeleton-gtk.h

@@ -31,6 +31,10 @@
 #define INIT_LIBNOTIFY FALSE
 #endif
 
+#ifndef PRE_GTK_INIT
+#define PRE_GTK_INIT()
+#endif
+
 #define GNOME_SESSION_DBUS_NAME           "org.gnome.SessionManager"
 #define GNOME_SESSION_DBUS_PATH           "/org/gnome/SessionManager"
 #define GNOME_SESSION_CLIENT_PRIVATE_NAME "org.gnome.SessionManager.ClientPrivate"
@@ -210,6 +214,8 @@ main (int argc, char **argv)
         g_type_ensure (G_TYPE_DBUS_CONNECTION);
         g_type_ensure (G_TYPE_DBUS_PROXY);
 
+        PRE_GTK_INIT();
+
         if (FORCE_X11_BACKEND) {
             const gchar *setup_display = getenv ("GNOME_SETUP_DISPLAY");
             if (setup_display && *setup_display != '\0')

plugins/common/meson.build

@@ -12,6 +12,11 @@ common_deps = [
     xi,
 ]
 
+if gtk_layer_shell_enabled
+    common_sources += 'wayland-utils.c'
+    common_deps += wayland_client
+endif
+
 common_inc = include_directories('.')
 
 common_lib = static_library(

plugins/common/wayland-utils.c

@@ -0,0 +1,70 @@
+/* wayland-utils.c - Wayland utility functions for CSD plugins
+ *
+ * Copyright (C) 2026 Linux Mint
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2 of the License, or
+ * (at your option) any later version.
+ */
+
+#include "config.h"
+#include "wayland-utils.h"
+
+#include <wayland-client.h>
+
+static gboolean layer_shell_available = FALSE;
+
+static void
+registry_handle_global (void                *data,
+                        struct wl_registry  *registry,
+                        uint32_t             name,
+                        const char          *interface,
+                        uint32_t             version)
+{
+    if (g_strcmp0 (interface, "zwlr_layer_shell_v1") == 0)
+        layer_shell_available = TRUE;
+}
+
+static void
+registry_handle_global_remove (void               *data,
+                               struct wl_registry *registry,
+                               uint32_t            name)
+{
+}
+
+static const struct wl_registry_listener registry_listener = {
+    registry_handle_global,
+    registry_handle_global_remove,
+};
+
+/**
+ * csd_check_layer_shell_support:
+ *
+ * Check if wlr-layer-shell protocol is supported by connecting to the
+ * Wayland display and checking the registry. This can be called before
+ * gtk_init() unlike gtk_layer_is_supported().
+ *
+ * Returns: %TRUE if layer shell is available, %FALSE otherwise
+ */
+gboolean
+csd_check_layer_shell_support (void)
+{
+    struct wl_display *display;
+    struct wl_registry *registry;
+
+    layer_shell_available = FALSE;
+
+    display = wl_display_connect (NULL);
+    if (!display)
+        return FALSE;
+
+    registry = wl_display_get_registry (display);
+    wl_registry_add_listener (registry, &registry_listener, NULL);
+    wl_display_roundtrip (display);
+
+    wl_registry_destroy (registry);
+    wl_display_disconnect (display);
+
+    return layer_shell_available;
+}

plugins/common/wayland-utils.h

@@ -0,0 +1,22 @@
+/* wayland-utils.h - Wayland utility functions for CSD plugins
+ *
+ * Copyright (C) 2026 Linux Mint
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2 of the License, or
+ * (at your option) any later version.
+ */
+
+#ifndef __WAYLAND_UTILS_H
+#define __WAYLAND_UTILS_H
+
+#include <glib.h>
+
+G_BEGIN_DECLS
+
+gboolean csd_check_layer_shell_support (void);
+
+G_END_DECLS
+
+#endif /* __WAYLAND_UTILS_H */