Fix double-click file opening in installed Windows app (v1.7.5)

This fix addresses the critical issue where double-clicking .md files would not open them in the installed PanConverter application on Windows.

## Root Cause
When a user double-clicks a file and the app is already running, Windows tries to launch a SECOND INSTANCE with the file path. Without single-instance lock handling, that second instance would exit and the first instance would never receive the file path.

## Changes
- Added single-instance lock handling in main.js (lines 52-85)
- Implemented second-instance event handler to capture file paths
- Focus existing window when second instance is attempted
- Pass file path to existing instance using openFileFromPath()
- Properly handle rendererReady state for file opening

## Testing
- Works in development mode (npm start)
- Should now work correctly in installed Windows app with double-click
- Maintains single instance behavior across all file opening methods

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

package.json

@@ -1,6 +1,6 @@
 {
   "name": "pan-converter",
-  "version": "1.7.3",
+  "version": "1.7.5",
   "description": "Cross-platform Markdown editor and converter using Pandoc",
   "main": "src/main.js",
   "scripts": {

src/main.js

@@ -49,6 +49,41 @@ let currentFile = null; // This will now represent the active tab's file
 let pandocAvailable = null; // Cache pandoc availability check
 let rendererReady = false; // Track if renderer is ready to receive file data
 
+// Handle single instance lock for Windows file association
+// When a file is double-clicked and the app is already running,
+// Windows tries to start a second instance. We prevent this and
+// pass the file to the existing instance instead.
+const gotTheLock = app.requestSingleInstanceLock();
+
+if (!gotTheLock) {
+  // Another instance is already running, quit this one
+  app.quit();
+} else {
+  // This is the first instance, handle second-instance events
+  app.on('second-instance', (event, commandLine, workingDirectory) => {
+    // Someone tried to run a second instance, focus our window instead
+    if (mainWindow) {
+      if (mainWindow.isMinimized()) mainWindow.restore();
+      mainWindow.focus();
+    }
+
+    // Check if a file was passed to the second instance
+    // commandLine is an array like: ['electron.exe', 'app.asar', 'file.md']
+    const fileArgs = commandLine.slice(2);
+    for (const arg of fileArgs) {
+      if ((arg.endsWith('.md') || arg.endsWith('.markdown')) && fs.existsSync(arg)) {
+        // Open the file in the existing instance
+        if (rendererReady) {
+          openFileFromPath(arg);
+        } else {
+          app.pendingFile = arg;
+        }
+        break;
+      }
+    }
+  });
+}
+
 // Check if pandoc is available
 function checkPandocAvailability() {
   return new Promise((resolve) => {
@@ -410,7 +445,7 @@ function createMenu() {
               type: 'info',
               title: 'About PanConverter',
               message: 'PanConverter',
-              detail: 'A cross-platform Markdown editor and converter using Pandoc.\n\nVersion: 1.7.3\nAuthor: Amit Haridas\nEmail: amit.wh@gmail.com\nLicense: MIT\n\nFeatures:\n• Modern glassmorphism UI with gradient backgrounds\n• Comprehensive PDF Editor (merge, split, compress, rotate, watermark, encrypt)\n• Universal File Converter (LibreOffice, ImageMagick, FFmpeg, Pandoc)\n• Windows Explorer context menu integration\n• Tabbed interface for multiple files\n• Advanced markdown editing with live preview\n• Real-time preview updates while typing\n• Full toolbar markdown editing functions\n• Enhanced PDF export with built-in Electron fallback\n• File association support for .md files\n• Command-line interface for batch conversion\n• Advanced export options with templates and metadata\n• Batch file conversion with progress tracking\n• Improved preview typography and spacing\n• Adjustable font sizes via menu (Ctrl+Shift+Plus/Minus)\n• Complete theme support including Monokai fixes\n• Find & replace with match highlighting\n• Line numbers and auto-indentation\n• Export to multiple formats via Pandoc\n• PowerPoint & presentation export\n• Export tables to Excel/ODS spreadsheets\n• Document import & conversion\n• Table creation helper\n• 22 beautiful themes (including Dracula, Nord, Tokyo Night, Gruvbox, Ayu, Concrete, and more)\n• Undo/redo functionality\n• Live word count and statistics',
+              detail: 'A cross-platform Markdown editor and converter using Pandoc.\n\nVersion: 1.7.4\nAuthor: Amit Haridas\nEmail: amit.wh@gmail.com\nLicense: MIT\n\nFeatures:\n• Modern glassmorphism UI with gradient backgrounds\n• Comprehensive PDF Editor (merge, split, compress, rotate, watermark, encrypt)\n• Universal File Converter (LibreOffice, ImageMagick, FFmpeg, Pandoc)\n• Windows Explorer context menu integration\n• Tabbed interface for multiple files\n• Advanced markdown editing with live preview\n• Real-time preview updates while typing\n• Full toolbar markdown editing functions\n• Enhanced PDF export with built-in Electron fallback\n• File association support for .md files\n• Command-line interface for batch conversion\n• Advanced export options with templates and metadata\n• Batch file conversion with progress tracking\n• Improved preview typography and spacing\n• Adjustable font sizes via menu (Ctrl+Shift+Plus/Minus)\n• Complete theme support including Monokai fixes\n• Find & replace with match highlighting\n• Line numbers and auto-indentation\n• Export to multiple formats via Pandoc\n• PowerPoint & presentation export\n• Export tables to Excel/ODS spreadsheets\n• Document import & conversion\n• Table creation helper\n• 22 beautiful themes (including Dracula, Nord, Tokyo Night, Gruvbox, Ayu, Concrete, and more)\n• Undo/redo functionality\n• Live word count and statistics',
               buttons: ['OK']
             });
           }
@@ -1583,16 +1618,12 @@ function openFileFromPath(filePath) {
   if (fs.existsSync(filePath)) {
     currentFile = filePath;
     const content = fs.readFileSync(filePath, 'utf-8');
-    if (mainWindow && mainWindow.webContents) {
-      console.log('Attempting to open file:', filePath, 'rendererReady:', rendererReady);
-      // Always try to send, the renderer will handle it when ready
+    if (mainWindow && mainWindow.webContents && rendererReady) {
       mainWindow.webContents.send('file-opened', { path: filePath, content });
     } else {
-      // Store file to open after window is created
+      // Store file to open after renderer is ready
       app.pendingFile = filePath;
     }
-  } else {
-    console.error('File does not exist:', filePath);
   }
 }