Tray: initialise GTK and run its main loop on Linux

tray-icon needs gtk::init() and a running GTK loop on Linux, which
eframe/winit never provide, so the tray panicked at startup with "GTK has
not been initialized". Build the tray on a dedicated GTK thread instead.

Co-Authored-By: Claude <noreply@anthropic.com>

Cargo.lock

@@ -6329,6 +6329,7 @@ version = "0.1.0"
 dependencies = [
  "eframe",
  "egui-phosphor",
+ "gtk",
  "tokio",
  "tracing",
  "tracing-subscriber",

crates/tsunagi-tray/Cargo.toml

@@ -26,6 +26,11 @@ tracing.workspace = true
 tracing-subscriber.workspace = true
 egui-phosphor = "0.9"
 
+# Linux only: tray-icon (already GTK-based there) needs the app to initialise
+# GTK and run its main loop, which eframe/winit do not do.
+[target.'cfg(target_os = "linux")'.dependencies]
+gtk = "0.18"
+
 # macOS only: set the app's activation policy to Accessory so it lives in the
 # menu bar with no Dock icon. Pinned to eframe's winit so the types unify.
 [target.'cfg(target_os = "macos")'.dependencies]

crates/tsunagi-tray/src/tray.rs

@@ -8,7 +8,7 @@
 use std::sync::mpsc;
 
 use eframe::egui;
-use tray_icon::menu::{Menu, MenuEvent, MenuItem, PredefinedMenuItem};
+use tray_icon::menu::{Menu, MenuEvent, MenuId, MenuItem, PredefinedMenuItem};
 use tray_icon::{Icon, TrayIcon, TrayIconBuilder};
 
 /// What the user picked from the tray menu.
@@ -22,34 +22,81 @@ pub(crate) enum Action {
 /// The live tray icon plus the channel of menu actions.
 pub(crate) struct Tray {
     /// Kept alive for as long as the app runs; dropping it removes the icon.
+    /// On Linux the icon lives on the GTK thread instead (it is not `Send`).
+    #[cfg(not(target_os = "linux"))]
     _icon: TrayIcon,
     actions: mpsc::Receiver<Action>,
 }
 
+/// A built tray icon and the ids of its menu items.
+struct Built {
+    icon: TrayIcon,
+    open: MenuId,
+    quit: MenuId,
+}
+
+fn build() -> Result<Built, String> {
+    let open = MenuItem::new("Open tsunagi", true, None);
+    let quit = MenuItem::new("Quit", true, None);
+    let menu = Menu::new();
+    menu.append(&open).map_err(|err| err.to_string())?;
+    menu.append(&PredefinedMenuItem::separator())
+        .map_err(|err| err.to_string())?;
+    menu.append(&quit).map_err(|err| err.to_string())?;
+
+    let icon = TrayIconBuilder::new()
+        .with_tooltip("tsunagi")
+        .with_menu(Box::new(menu))
+        .with_icon(make_icon()?)
+        .build()
+        .map_err(|err| err.to_string())?;
+    Ok(Built {
+        icon,
+        open: open.id().clone(),
+        quit: quit.id().clone(),
+    })
+}
+
 impl Tray {
     /// Builds the tray and starts reading its menu events.
     ///
-    /// Created on the main thread (required on macOS); `ctx` is used to wake the
-    /// UI when a menu item is chosen.
+    /// Created on the main thread on macOS and Windows (required on macOS). On
+    /// Linux the tray needs GTK, which eframe/winit never initialise, so it is
+    /// built on a dedicated thread that initialises GTK and runs its main loop.
+    /// `ctx` is used to wake the UI when a menu item is chosen.
     pub(crate) fn new(ctx: egui::Context) -> Result<Self, String> {
-        let open = MenuItem::new("Open tsunagi", true, None);
-        let quit = MenuItem::new("Quit", true, None);
-        let menu = Menu::new();
-        menu.append(&open).map_err(|err| err.to_string())?;
-        menu.append(&PredefinedMenuItem::separator())
-            .map_err(|err| err.to_string())?;
-        menu.append(&quit).map_err(|err| err.to_string())?;
+        #[cfg(target_os = "linux")]
+        let (open_id, quit_id) = {
+            let (ready_tx, ready_rx) = mpsc::sync_channel(1);
+            std::thread::Builder::new()
+                .name("tray-gtk".into())
+                .spawn(move || {
+                    if let Err(err) = gtk::init() {
+                        let _ = ready_tx.send(Err(format!("cannot initialise GTK: {err}")));
+                        return;
+                    }
+                    match build() {
+                        Ok(built) => {
+                            let _ = ready_tx.send(Ok((built.open, built.quit)));
+                            gtk::main();
+                            drop(built.icon);
+                        }
+                        Err(err) => {
+                            let _ = ready_tx.send(Err(err));
+                        }
+                    }
+                })
+                .map_err(|err| err.to_string())?;
+            ready_rx
+                .recv()
+                .map_err(|_| "the tray thread exited before it was ready".to_string())??
+        };
+        #[cfg(not(target_os = "linux"))]
+        let (icon, open_id, quit_id) = {
+            let built = build()?;
+            (built.icon, built.open, built.quit)
+        };
 
-        let icon = make_icon()?;
-        let tray = TrayIconBuilder::new()
-            .with_tooltip("tsunagi")
-            .with_menu(Box::new(menu))
-            .with_icon(icon)
-            .build()
-            .map_err(|err| err.to_string())?;
-
-        let open_id = open.id().clone();
-        let quit_id = quit.id().clone();
         let (tx, rx) = mpsc::channel();
         std::thread::spawn(move || {
             // The global menu-event channel is fed by the OS event loop.
@@ -69,7 +116,8 @@ impl Tray {
         });
 
         Ok(Self {
-            _icon: tray,
+            #[cfg(not(target_os = "linux"))]
+            _icon: icon,
             actions: rx,
         })
     }