Avoid panicking on X11 attention hint send failure

src/platform_impl/linux/x11/window.rs

@@ -1814,9 +1814,11 @@ impl UnownedWindow {
                 .unwrap_or_default();
 
         wm_hints.urgent = request_type.is_some();
-        wm_hints
-            .set(self.xconn.xcb_connection(), self.xwindow as xproto::Window)
-            .expect_then_ignore_error("Failed to set WM hints");
+        if let Ok(cookie) =
+            wm_hints.set(self.xconn.xcb_connection(), self.xwindow as xproto::Window)
+        {
+            cookie.ignore_error();
+        }
     }
 
     #[inline]

tests/x11_user_attention.rs

@@ -0,0 +1,74 @@
+#![cfg(all(target_os = "linux", feature = "x11"))]
+
+use std::io::{BufRead, BufReader};
+use std::process::{Command, Stdio};
+use std::time::Duration;
+
+use winit::application::ApplicationHandler;
+use winit::event::WindowEvent;
+use winit::event_loop::{ActiveEventLoop, EventLoop};
+use winit::platform::pump_events::EventLoopExtPumpEvents;
+use winit::platform::x11::EventLoopBuilderExtX11;
+use winit::window::{UserAttentionType, Window, WindowId};
+
+#[derive(Default)]
+struct App {
+    window: Option<Window>,
+}
+
+impl ApplicationHandler for App {
+    fn resumed(&mut self, event_loop: &ActiveEventLoop) {
+        self.window = Some(event_loop.create_window(Window::default_attributes()).unwrap());
+    }
+
+    fn window_event(
+        &mut self,
+        _event_loop: &ActiveEventLoop,
+        _window_id: WindowId,
+        _event: WindowEvent,
+    ) {
+    }
+}
+
+#[test]
+fn requesting_attention_after_x11_disconnect_does_not_panic() {
+    let mut server = match Command::new("Xvfb")
+        .args(["-displayfd", "1", "-nolisten", "tcp"])
+        .stdout(Stdio::piped())
+        .stderr(Stdio::null())
+        .spawn()
+    {
+        Ok(server) => server,
+        Err(error) if error.kind() == std::io::ErrorKind::NotFound => return,
+        Err(error) => panic!("failed to start Xvfb: {error}"),
+    };
+
+    let mut display = String::new();
+    BufReader::new(server.stdout.take().unwrap()).read_line(&mut display).unwrap();
+    let mut builder = EventLoop::builder();
+    builder.with_x11().with_any_thread(true);
+    let mut event_loop = {
+        let previous_display = std::env::var_os("DISPLAY");
+        std::env::set_var("DISPLAY", format!(":{}", display.trim()));
+        let result = builder.build().unwrap();
+        if let Some(previous_display) = previous_display {
+            std::env::set_var("DISPLAY", previous_display);
+        } else {
+            std::env::remove_var("DISPLAY");
+        }
+        result
+    };
+
+    let mut app = App::default();
+    event_loop.pump_app_events(Some(Duration::ZERO), &mut app);
+    let window = app.window.take().unwrap();
+    window.request_user_attention(Some(UserAttentionType::Informational));
+
+    server.kill().unwrap();
+    server.wait().unwrap();
+    window.request_user_attention(Some(UserAttentionType::Informational));
+    // Xlib may try to use the dead connection while destroying the window and event loop.
+
+    std::mem::forget(window);
+    std::mem::forget(event_loop);
+}