READU.md

A fast, lightweight Markdown reader & editor for Windows, built with Fluent Design.

⬇️ Download Latest Release — Extract & run, no installation needed.

🛍️ Get it on Microsoft Store

Screenshots

Features & TOC	Mermaid Diagrams

Dark Mode	Edit Mode (Split View)

Features

• Multi-Tab — open multiple files simultaneously, with drag-to-reorder tabs
• Edit Mode — side-by-side Markdown editor + live preview (Ctrl+E)
• Instant Rendering — powered by WebView2 (Chromium) and Markdig
• Syntax Highlighting — code blocks highlighted via highlight.js
• Mermaid.js — flowcharts, sequence diagrams, Gantt charts, and more
• Smart Mermaid Caching — SHA256-based incremental DOM updates preserve rendered diagrams
• Dark Mode — automatically follows your Windows system theme
• Table of Contents — auto-generated sidebar with smooth scroll navigation
• Drag & Drop — drop any .md file to open it instantly
• Hot Reload — automatically refreshes when the file changes externally
• Scroll & Zoom Persistence — per-tab scroll position and zoom level preserved across tab switches
• Print / PDF Export — Ctrl+P to print or save as PDF
• Full Page Screenshot — capture the entire rendered page as PNG (Ctrl+Shift+S)
• File Association — double-click .md files to open in READU.md
• Lightweight — minimal memory footprint, instant startup with pre-loaded CDN shell

Optional AI Summary

AI Summary is optional and does not change the default READU.md experience for users who just want a Markdown reader/editor.

• Currently validated with Azure OpenAI-compatible endpoints and gpt-5.4-mini
• If you run into issues with another AI provider, please open an issue with the details

Keyboard Shortcuts

Shortcut	Action
Ctrl+O	Open file (new tab)
Ctrl+W	Close current tab
Ctrl+Tab	Next tab
Ctrl+Shift+Tab	Previous tab
Ctrl+E	Toggle Edit / Read mode
Ctrl+S	Save file (edit mode)
Ctrl+N	New blank tab (edit mode)
Ctrl+P	Print / Export PDF
Ctrl+Shift+S	Full page screenshot
Ctrl+ + / -	Zoom in / out
Ctrl+0	Reset zoom to 100%
Ctrl+Home	Open Welcome page

Tech Stack

Component	Technology
Framework	.NET 9
UI	WinUI 3 (Windows App SDK 1.6)
Markdown	Markdig
Rendering	WebView2 (Chromium)
Syntax Highlighting	highlight.js (CDN)
Diagrams	Mermaid.js v11 (CDN)
Window Management	WinUIEx

Architecture & Trade-offs

READU.md is a WinUI 3 desktop shell that brings together several open-source libraries. Here's how the pieces fit and what trade-offs were made:

Markdown parsing is handled entirely by Markdig — a fast, extensible .NET Markdown processor. READU.md doesn't reimplement parsing; Markdig does the heavy lifting and does it well.

Rendering uses a WebView2 (Chromium) control. This was a deliberate trade-off: a native text renderer would use less memory, but wouldn't support Mermaid diagrams, LaTeX math, or the full CSS styling that makes Markdown readable. WebView2 is pre-installed on Windows 10/11, so there's no extra download.

Shell page architecture — CDN resources (highlight.js, mermaid.js, CSS) are loaded into the WebView2 once at startup. When switching tabs or updating content, only the HTML body is injected via JavaScript (updateContent()), avoiding full page reloads. This is what makes tab-switching and edit-mode preview fast.

Mermaid diagram caching — each Mermaid code block is hashed (SHA256). During live preview updates, unchanged diagrams keep their rendered SVGs instead of being re-rendered — this prevents the "flash" you'd normally see.

What this project adds on top of these libraries:

• Native WinUI 3 app with system theme integration (Mica, dark/light mode)
• Multi-tab document management with per-tab state (scroll, zoom, edit mode)
• File watching with debounced hot-reload
• Full-page screenshot via CDP (Page.captureScreenshot)
• File association and drag-and-drop integration

Build

Prerequisites

• .NET 9 SDK
• Visual Studio 2022 17.12+ with Windows application development workload
• Windows 10 (1903+) or Windows 11

Build & Run

# Clone
git clone https://github.com/breezy89757/READU.md.git
cd READU.md

# Build
dotnet build src\READU.md.csproj -p:Platform=x64

# Or open READU.md.sln in Visual Studio and press F5

Settings

Settings are stored at %LOCALAPPDATA%\READU.md\settings.json:

{
  "properties": {
    "enable_mermaid": { "value": true },
    "font_size": { "value": 14 }
  }
}

Contributing

Contributions are welcome! Please open an issue to discuss your idea before submitting a PR.

License

MIT