feat: enforce singleton via tauri-plugin-single-instance (v6.4.3)

Add `tauri-plugin-single-instance` to prevent multiple copies of Display
DJ from running at the same time. Without this, autostart racing a
manual launch (or double-clicking the .app) could result in two tray
icons and two competing sidecar processes. The plugin is registered
first in the builder so the lock check happens before any sidecar spawn
or tray setup; the duplicate process exits immediately and the running
instance just logs the duplicate launch (the app is tray-only, so there
is no main window to refocus).

Co-Authored-By: Claude Opus 4 (1M context) <noreply@anthropic.com>

Author: synle

README.md

@@ -50,6 +50,7 @@ A cross-platform desktop system tray app for controlling monitor brightness, con
 - **Settings panel** -- tabbed UI (General + Tiling) with auto-save. Configure brightness, contrast, monitors, night mode, snap zones, exposé grid size, layout strategy, and launch at login
 - **About / Update check** -- "About Display DJ" in the tray menu shows current version, latest GitHub release, engine, platform, build date, and homepage. Green "Up to date" or orange "Update available" badge with download link. macOS section includes quarantine fix and Accessibility settings commands
 - **System tray app** -- lives in your menu bar / system tray with no dock or taskbar clutter
+- **Single-instance** -- only one copy of Display DJ runs at a time. If you accidentally launch it twice (e.g. autostart racing a manual launch), the duplicate exits silently so you never end up with multiple tray icons or competing sidecars
 
 ## Download & Install
 

src-tauri/Cargo.lock

@@ -18,6 +18,7 @@ tauri = { version = "2", features = ["tray-icon"] }
 tauri-plugin-autostart = "2"
 tauri-plugin-global-shortcut = "2"
 tauri-plugin-shell = "2"
+tauri-plugin-single-instance = "2"
 serde = { version = "1", features = ["derive"] }
 serde_json = "1"
 dirs = "6"

src-tauri/Cargo.toml

@@ -417,6 +417,24 @@ mod tests {
         assert!(!is_night_time(420, 420, 420));
         assert!(!is_night_time(420, 420, 1000));
     }
+
+    // -- single-instance plugin --
+
+    /// Verifies that `tauri_plugin_single_instance::init` is callable with the
+    /// callback signature we use in `run()`. This guards against accidental
+    /// removal of the dependency or signature drift in plugin upgrades. We
+    /// build the plugin without registering it with a Tauri builder, which is
+    /// safe because `init` itself doesn't spawn anything — the singleton lock
+    /// is acquired only when the plugin is registered with `Builder::plugin`.
+    #[test]
+    fn test_single_instance_plugin_callback_signature() {
+        let _plugin = tauri_plugin_single_instance::init(
+            |_app: &tauri::AppHandle, _args: Vec<String>, _cwd: String| {
+                // Callback body intentionally empty — we only verify the
+                // signature compiles and the plugin can be constructed.
+            },
+        );
+    }
 }
 
 /// Main entry point: builds the Tauri app, spawns the display-dj sidecar,
@@ -427,6 +445,21 @@ pub fn run() {
     let preferences = config::load_preferences();
 
     tauri::Builder::default()
+        // Singleton enforcement: if a second copy of Display DJ is launched, the
+        // plugin notifies this (the first) instance via the callback below and
+        // the second process exits immediately. This prevents duplicate tray
+        // icons and conflicting sidecar processes when the app is launched
+        // multiple times (e.g. user double-clicks .app, autostart races a
+        // manual launch, etc.). The callback receives the second instance's
+        // CLI args + cwd; we only log them since the app is tray-only and has
+        // no main window to refocus.
+        .plugin(tauri_plugin_single_instance::init(|_app, args, cwd| {
+            log::info!(
+                "single-instance: ignored duplicate launch (args={:?}, cwd={:?})",
+                args,
+                cwd,
+            );
+        }))
         .plugin(tauri_plugin_shell::init())
         .plugin(tauri_plugin_dialog::init())
         .plugin(tauri_plugin_global_shortcut::Builder::new().build())

src-tauri/src/lib.rs

@@ -1,7 +1,7 @@
 {
   "$schema": "https://raw.githubusercontent.com/nicoverbruggen/tauri-v2-docs/refs/heads/main/schemas/config.schema.json",
   "productName": "Display DJ",
-  "version": "6.4.2",
+  "version": "6.4.3",
   "identifier": "com.synle.display-dj",
   "build": {
     "beforeDevCommand": "npm run dev",