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+# ROBrowser Legacy - GitHub Copilot Instructions
+
+ROBrowser Legacy is a web-based MMORPG client for Ragnarok Online, built with JavaScript, HTML5, and CSS. It's packaged and distributed as both a website and can be containerized with Docker.
+
+**ALWAYS reference these instructions first and fallback to search or bash commands only when you encounter unexpected information that does not match the info here.**
+
+## Working Effectively
+
+### Prerequisites and Environment Setup
+- Install Node.js LTS (v18 or higher, tested with v20.19.4):
+  ```bash
+  node --version  # Should show v18.0.0 or higher
+  npm --version   # Should show compatible npm version
+  ```
+- Install dependencies:
+  ```bash
+  npm install  # Takes ~30 seconds. NEVER CANCEL.
+  ```
+
+### Build Commands - CRITICAL TIMING INFORMATION
+**NEVER CANCEL ANY BUILD COMMAND** - Set appropriate timeouts and wait for completion.
+
+- **Quick HTML build** (for testing):
+  ```bash
+  npm run build:html  # Takes <1 second
+  ```
+
+- **Individual module builds** (3-17 seconds each):
+  ```bash
+  npm run build:online        # Takes ~5 seconds - main game client
+  npm run build:mapviewer     # Takes ~10 seconds
+  npm run build:grfviewer     # Takes ~6 seconds  
+  npm run build:modelviewer   # Takes ~13 seconds
+  npm run build:strviewer     # Takes ~20 seconds
+  npm run build:effectviewer  # Takes ~24 seconds
+  npm run build:threadhandler # Takes ~24 seconds
+  ```
+
+- **Full build** (for production):
+  ```bash
+  npm run build:all  # Takes ~25 seconds. NEVER CANCEL. Set timeout to 60+ seconds.
+  ```
+
+- **Minified build** (for final production):
+  ```bash
+  npm run build:all:minify  # Takes ~3 minutes. NEVER CANCEL. Set timeout to 300+ seconds.
+  ```
+
+### Development and Testing
+- **Development server** (live reload):
+  ```bash
+  npm run live  # Serves on http://127.0.0.1:8080, opens browser examples
+  ```
+
+- **Production server** (serve built files):
+  ```bash
+  npm run serve  # Serves dist/Web on http://127.0.0.1:8080
+  ```
+
+- **Tools server** (development utilities):
+  ```bash
+  npm run tools  # Opens file testing and development tools
+  ```
+
+## Validation and Testing
+
+### Manual Validation Requirements
+**ALWAYS manually validate any new code by running through complete user scenarios** after making changes:
+
+1. **Build validation**:
+   ```bash
+   npm run build:all  # Must complete without errors
+   npm run serve      # Must start server successfully
+   ```
+
+2. **Application validation**:
+   - Navigate to http://127.0.0.1:8080/
+   - Verify the Ragnarok Online client interface loads
+   - Check browser console (F12) for JavaScript errors
+   - Ensure the "START NOW" button and file drag-drop area are functional
+
+3. **File testing tools** (for game asset validation):
+   - Access via `npm run tools`
+   - Use applications/tools/tests/ for testing game file formats
+   - Available testers: sprites, models, altitude, ground, str, targa, action files
+
+### No Traditional Testing Framework
+- **NO** Jest, Mocha, or similar unit test frameworks
+- **NO** configured linters (ESLint, JSHint, etc.)
+- Custom file testing tools in `applications/tools/tests/`
+- Code style enforced via `.editorconfig` (tabs, UTF-8, etc.)
+
+## Repository Structure and Navigation
+
+### Key Directories
+```
+├── src/                    # Main source code
+│   ├── Core/              # Core engine (Client, Thread, Events, Configs)
+│   ├── UI/Components/     # User interface components
+│   ├── Engine/            # Game engine (EntityManager, MapEngine)
+│   ├── Network/           # Network communication
+│   ├── Renderer/          # WebGL rendering
+│   ├── Audio/             # Audio system
+│   ├── Loaders/           # Asset loaders (GRF, sprites, models)
+│   ├── Plugins/           # Plugin system
+│   └── Vendors/           # Third-party libraries
+├── applications/          # Built applications and tools
+│   ├── browser-examples/  # Demo and example pages
+│   ├── tools/            # Build tools and file testers
+│   └── nwjs/             # NW.js desktop app
+├── dist/Web/             # Built output directory
+└── doc/                  # Documentation and setup guides
+```
+
+### Important Files to Know
+- `package.json` - Build scripts and dependencies
+- `applications/tools/builder-web.js` - Main build system
+- `src/Core/Client.js` - Main game client
+- `src/Core/Configs.js` - Configuration system
+- `src/Plugins/PluginManager.js` - Plugin loading system
+- `index.html` - Main application entry point
+
+## Common Development Tasks
+
+### Making Code Changes
+1. **ALWAYS** build and test before making changes:
+   ```bash
+   npm run build:all && npm run serve
+   ```
+
+2. **For UI changes**: Test with the development server:
+   ```bash
+   npm run live  # Live reload for faster iteration
+   ```
+
+3. **For plugin development**: 
+   - Add plugins to `src/Plugins/`
+   - Configure in game settings or via config file
+   - Reference existing plugins for structure
+
+4. **For core changes**: Focus on these key areas:
+   - `src/Core/` - Engine functionality
+   - `src/UI/Components/` - User interface
+   - `src/Network/` - Server communication
+   - `src/Engine/` - Game logic
+
+### Build Artifacts and Distribution
+- **Development**: Use `npm run live` with source files
+- **Production web**: Use `npm run build:all` → `dist/Web/`
+- **Production optimized**: Use `npm run build:all:minify` → `dist/Web/`
+- **Docker**: Use provided Dockerfile (builds automatically)
+
+### Troubleshooting Common Issues
+1. **Build failures**: Check Node.js version (must be v18+)
+2. **Asset loading errors**: Verify game files are properly placed
+3. **Blank screen**: Check browser console for JavaScript errors
+4. **Network errors**: Ensure WebSocket proxy is configured
+5. **Performance issues**: Use minified build for production
+
+## Docker Support
+- **Development container**: `docker-compose up ro-browser`
+- **Production container**: `docker-compose up serve-dist`
+- **Custom build**: See Dockerfile for production setup
+
+## Critical Reminders
+- **NEVER CANCEL** build commands - they may take several minutes
+- **ALWAYS** test functionality manually after code changes
+- **NO** traditional unit tests - use file testing tools and manual validation
+- **NO** configured linters - follow `.editorconfig` style guidelines
+- Set build timeouts to **60+ seconds** for regular builds, **300+ seconds** for minified builds
+- Builds over 2 minutes are NORMAL for minified versions
+
+## External Dependencies
+- **WebSocket Proxy**: Required for server connection (roBrowserLegacy-wsProxy)
+- **Remote Client**: Optional for centralized asset serving (PHP or JS implementations)  
+- **Game Assets**: GRF files required for full functionality
+- **Game Server**: Compatible Ragnarok Online server for online play
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