A clean, cross-platform (Windows / macOS) Markdown editor built on Electron.

Develop a Markdown editor that works on both Windows and Mac, supporting Markdown content editing, Markdown preview, and file open/save functions. Double-clicking a .md file in the system should open it directly in this app. The interface should be clean and simple.

Enhancements needed: replace the textarea with CodeMirror for syntax highlighting, add code block highlighting (highlight.js), and support exporting to HTML/PDF. Toolbar buttons should use intuitive icons. Add internationalization support — the app defaults to English and can be switched to Chinese, Japanese, French, Korean, or German. Support System/Dark/Light mode switching.

• CodeMirror editor: Markdown syntax highlighting, automatic list continuation, Tab indentation
• Live preview with code block highlighting via highlight.js (Edit / Split / Preview-only views, switch with Ctrl+1/2/3)
• New file (Ctrl+N), Open (Ctrl+O), Save (Ctrl+S), Save As (Ctrl+Shift+S)
• Export to HTML (Ctrl+E) / Export to PDF (Ctrl+Shift+E), exported content includes code highlighting styles
• Icon toolbar + status bar (file path, word count, export notifications)
• Internationalization: defaults to English, switchable to 中文 / 日本語 / Français / 한국어 / Deutsch (menus, dialogs, and UI all follow the selected language, with the choice persisted)
• Themes: System (follows OS) / Light / Dark, with coordinated colors for the editor and preview
• Double-click .md / .markdown files in the system to open directly in this app (installer build only)
• Unsaved changes protection: prompts to save before close, new, or open
• Single-instance mode: double-clicking a file again reuses the already-open window

npm install
npm start

npm run dist:win   # Windows, generates an NSIS installer (in the dist/ directory)
npm run dist:mac   # macOS, generates a dmg (must be run on a Mac)

Notes:

• .md double-click association only works in the "installer" build (npm start in dev mode does not register the file association). The association is configured via build.fileAssociations in package.json and is written automatically to the Windows registry / macOS Info.plist when packaged with electron-builder.
• The macOS dmg must be packaged on a Mac; unsigned apps require allowing them in "System Settings → Privacy & Security" on first launch.

main.js              Main process: window, i18n menus, file dialogs, HTML/PDF export, file association, single instance
preload.js           contextBridge security bridge + highlight.js highlighting service
locales.js           Six-language dictionary (shared by main and renderer processes)
renderer/index.html  UI layout (icon toolbar, CodeMirror, theme/language switching)
renderer/app.js      Editor logic, live preview (marked + hljs), i18n, theme switching
renderer/style.css   App styles (CSS variables for light/dark themes)
renderer/export.css  Standalone styles for HTML/PDF export
sample.md            Test document

Technical notes:

• System theme mode follows the OS via matchMedia('(prefers-color-scheme: dark)') and listens for changes; in dark mode, CodeMirror switches to the material-darker theme and the preview switches to the github-dark highlighting style.
• Language/theme selections are stored in localStorage; when the language changes, the renderer notifies the main process to rebuild the application menu.
• The Undo/Redo/Select All menu items do not use system roles (which would act on CodeMirror's hidden textarea); they are forwarded to CodeMirror instead.
• PDF export: the main process writes the rendered HTML to a temporary file, loads it in a hidden BrowserWindow, and calls printToPDF (A4, with background colors).
• The preload script needs require('highlight.js'), so the window disables Electron's default preload sandbox (sandbox: false, while still keeping contextIsolation enabled).

MIT — use it freely for both perosnal or commercial.