SpotlightDimmer

A Windows utility that creates semi-transparent overlays to dim inactive displays or regions, creating a "spotlight" effect on the active window. Built with .NET 10 and native Windows APIs for optimal performance.

Features

✅ Multi-monitor support - Automatically detects all connected monitors ✅ Configurable overlays - Customize colors and opacity for both inactive and active regions independently ✅ Multiple rendering backends - Choose between:

• LayeredWindow: Extremely lightweight (< 10MB RAM usage)
• CompositeOverlay: Better visual quality during window dragging (~50MB RAM for dual monitor setup with partial overlays) ✅ Small footprint - Less than 50MB installed, installer under 10MB ✅ Click-through overlays - Overlays don't capture mouse input (WS_EX_TRANSPARENT) ✅ 100% event-driven - Uses Windows event hooks instead of polling for zero CPU usage when idle ✅ No admin privileges - Runs as a regular user process ✅ Hot-reloadable configuration - Changes apply instantly without restart ✅ Native AOT compilation - Fast startup and minimal runtime dependencies

How It Works

The application uses a dual event hook system for comprehensive window tracking:

Event Hooks (100% Event-Driven - No Polling!)

• EVENT_SYSTEM_FOREGROUND - Instant detection when switching between applications
• EVENT_OBJECT_LOCATIONCHANGE - Real-time detection of window movement:
  • Detects windows being dragged between monitors with the mouse
  • Detects Win+Arrow and Win+Shift+Arrow keyboard shortcuts
  • Filters out cursor/caret events using OBJID_WINDOW check

Window Management APIs

• EnumDisplayMonitors - Detects all connected monitors
• CreateWindowEx - Creates overlay windows with layered and transparent styles
• SetLayeredWindowAttributes - Sets the semi-transparent appearance with configurable opacity

Building

Regular Build

dotnet build

Run

dotnet run

AOT Build (requires Visual Studio C++ tools)

dotnet publish -c Release -r win-x64

Architecture

• WinApi.cs - P/Invoke declarations for Windows APIs
• MonitorManager.cs - Multi-monitor detection and management
• OverlayWindow.cs - Semi-transparent, click-through overlay windows
• FocusTracker.cs - Event-driven focus tracking using Windows hooks
• Program.cs - Main application logic and message loop

Performance Notes

• 100% event-driven - No polling whatsoever!
• Zero CPU usage when idle - Only activates on actual window changes
• Instant response - Event hooks provide immediate notification of window changes
• Efficient movement detection - Tracks window position and focus changes in real-time
• Native Windows API calls for maximum performance
• AOT compilation eliminates JIT overhead and reduces startup time

Configuration

SpotlightDimmer can be configured via JSON file located at %AppData%\SpotlightDimmer\config.json. See CONFIGURATION.md for detailed configuration options.