nemo-preview: Force x11 backend for wayland sessions.

This plugin really needs a rewrite or retirement (such as
linuxmint/nemo#3650), but for now keep it working as an x11 client.

Author: mtwebster

nemo-preview/src/libnemo-preview/nemo-preview-utils.c

@@ -93,17 +93,20 @@ nemo_preview_create_rounded_background (void)
  * nemo_preview_create_foreign_window:
  * @xid:
  *
- * Returns: (transfer full): a #GdkWindow
+ * Returns: (transfer full) (nullable): a #GdkWindow, or %NULL if @xid can't be
+ * looked up. Nemo sends 0 when it has no X11 window to parent us to.
  */
 GdkWindow *
 nemo_preview_create_foreign_window (guint xid)
 {
-  GdkWindow *retval;
+  GdkDisplay *display = gdk_display_get_default ();
 
-  retval = gdk_x11_window_foreign_new_for_display (gdk_display_get_default (),
-                                                   xid);
+  /* gdk_x11_window_foreign_new_for_display() only checks for GDK_IS_DISPLAY,
+   * so a wayland display would be cast to a GdkX11Display and crash. */
+  if (xid == 0 || !GDK_IS_X11_DISPLAY (display))
+    return NULL;
 
-  return retval;
+  return gdk_x11_window_foreign_new_for_display (display, xid);
 }
 
 /**

nemo-preview/src/main.c

@@ -108,6 +108,11 @@ main (int argc, char **argv)
   GjsContext *js_context;
   GError *error;
 
+  /* Clutter, the ARGB visual and foreign window parenting are all X11 only,
+   * so run under XWayland rather than fail on a wayland display. */
+  g_setenv ("GDK_BACKEND", "x11", TRUE);
+  g_setenv ("CLUTTER_BACKEND", "x11", TRUE);
+
 #ifdef GDK_WINDOWING_X11
   XInitThreads ();
 #endif