focus existing window on port conflict, closes #3036

electron.js

@@ -29,7 +29,7 @@ app.on('ready', async () => {
     if (await sqlInit.isDbInitialized()) {
         await sqlInit.dbReady;
 
-        await windowService.createMainWindow();
+        await windowService.createMainWindow(app);
 
         tray.createTray();
     }

src/services/window.js

@@ -44,7 +44,7 @@ ipcMain.on('create-extra-window', (event, arg) => {
     createExtraWindow(arg.notePath, arg.hoistedNoteId);
 });
 
-async function createMainWindow() {
+async function createMainWindow(app) {
     const windowStateKeeper = require('electron-window-state'); // should not be statically imported
 
     const mainWindowState = windowStateKeeper({
@@ -81,6 +81,18 @@ async function createMainWindow() {
     mainWindow.on('closed', () => mainWindow = null);
 
     configureWebContents(mainWindow.webContents, spellcheckEnabled);
+
+    app.on('second-instance', () => {
+        // Someone tried to run a second instance, we should focus our window.
+        // see www.js "requestSingleInstanceLock" for the rest of this logic with explanation
+        if (mainWindow) {
+            if (mainWindow.isMinimized()) {
+                mainWindow.restore();
+            }
+
+            mainWindow.focus();
+        }
+    });
 }
 
 function configureWebContents(webContents, spellcheckEnabled) {

src/www

@@ -38,6 +38,21 @@ if (!semver.satisfies(process.version, ">=10.5.0")) {
 let httpServer;
 
 async function startTrilium() {
+    /**
+     * The intended behavior is to detect when a second instance is running, in that case open the old instance
+     * instead of the new one. This is complicated by the fact that it is possible to run multiple instances of Trilium
+     * if port and data dir is configured separately. This complication is the source of the following weird usage.
+     *
+     * The line below makes sure that the "second-instance" (process in window.js) is fired. Normally it returns a boolean
+     * indicating whether another instance is running or not, but we ignore that and kill the app only based on the port conflict.
+     *
+     * A bit weird is that "second-instance" is triggered also on the valid usecases (different port/data dir) and
+     * focuses the existing window. But the new process is start as well and will steal the focus too, it will win, because
+     * its startup is slower than focusing the existing process/window. So in the end it works out without having
+     * to do complex evaluation.
+     */
+    require("electron").app.requestSingleInstanceLock();
+
     const usedPort = await port;
     const usedHost = await host;