fix error

src-tauri/src/x11mod.rs

@@ -104,9 +104,12 @@ fn fetch_tauri_xid(app: &AppHandle, window: tauri::WebviewWindow) -> Result<u32,
             use gtk::prelude::*;
             let gtk_win = window.gtk_window().map_err(|e| e.to_string())?;
             let gdk_win = gtk_win.window().ok_or_else(|| "no GDK window yet".to_string())?;
-            let x11: gdkx11::X11Window = gdk_win
-                .downcast()
-                .map_err(|_| "GDK window is not an X11Window".to_string())?;
+            let x11: gdkx11::X11Window = gdk_win.downcast().map_err(|_| {
+                "Tauri's window is not an X11 window — likely running on Wayland. \
+                 GDK_BACKEND=x11 should force X11; check that XWayland is installed \
+                 (apt install xwayland) and restart the app."
+                    .to_string()
+            })?;
             Ok(x11.xid() as u32)
         })();
         let _ = tx.send(result);