Add platform::startup_notify for Wayland/X11

The utils in this module should help the users to activate the windows
they create, as well as manage activation tokens environment variables.

The API is essential for Wayland in the first place, since some
compositors may decide initial focus of the window based on whether
the activation token was during the window creation.

Fixes #2279.

Co-authored-by: John Nunley <jtnunley01@gmail.com>

src/platform_impl/linux/x11/window.rs

@@ -23,6 +23,7 @@ use x11rb::{
 use crate::{
     dpi::{PhysicalPosition, PhysicalSize, Position, Size},
     error::{ExternalError, NotSupportedError, OsError as RootOsError},
+    event_loop::AsyncRequestSerial,
     platform_impl::{
         x11::{atoms::*, MonitorHandle as X11MonitorHandle, X11Error},
         Fullscreen, MonitorHandle as PlatformMonitorHandle, OsError,
@@ -124,6 +125,7 @@ pub(crate) struct UnownedWindow {
     ime_sender: Mutex<ImeSender>,
     pub shared_state: Mutex<SharedState>,
     redraw_sender: Sender<WindowId>,
+    activation_sender: Sender<super::ActivationToken>,
 }
 
 impl UnownedWindow {
@@ -317,6 +319,7 @@ impl UnownedWindow {
             ime_sender: Mutex::new(event_loop.ime_sender.clone()),
             shared_state: SharedState::new(guessed_monitor, &window_attrs),
             redraw_sender: event_loop.redraw_sender.clone(),
+            activation_sender: event_loop.activation_sender.clone(),
         };
 
         // Title must be set before mapping. Some tiling window managers (i.e. i3) use the window
@@ -520,6 +523,11 @@ impl UnownedWindow {
             leap!(window.set_window_level_inner(window_attrs.window_level)).ignore_error();
         }
 
+        // Remove the startup notification if we have one.
+        if let Some(startup) = pl_attribs.activation_token.as_ref() {
+            leap!(xconn.remove_activation_token(xwindow, &startup._token));
+        }
+
         // We never want to give the user a broken window, since by then, it's too late to handle.
         let window = leap!(xconn.sync_with_server().map(|_| window));
 
@@ -1699,6 +1707,34 @@ impl UnownedWindow {
             .expect_then_ignore_error("Failed to set WM hints");
     }
 
+    #[inline]
+    pub(crate) fn generate_activation_token(&self) -> Result<String, X11Error> {
+        // Get the title from the WM_NAME property.
+        let atoms = self.xconn.atoms();
+        let title = {
+            let title_bytes = self
+                .xconn
+                .get_property(self.xwindow, atoms[_NET_WM_NAME], atoms[UTF8_STRING])
+                .expect("Failed to get title");
+
+            String::from_utf8(title_bytes).expect("Bad title")
+        };
+
+        // Get the activation token and then put it in the event queue.
+        let token = self.xconn.request_activation_token(&title)?;
+
+        Ok(token)
+    }
+
+    #[inline]
+    pub fn request_activation_token(&self) -> Result<AsyncRequestSerial, NotSupportedError> {
+        let serial = AsyncRequestSerial::get();
+        self.activation_sender
+            .send((self.id(), serial))
+            .expect("activation token channel should never be closed");
+        Ok(serial)
+    }
+
     #[inline]
     pub fn id(&self) -> WindowId {
         WindowId(self.xwindow as _)