Spotlight Dimmer

Overview

A lightweight Windows application that dims inactive displays to highlight the active one. Built with pure Rust and Windows API for maximum performance and minimal resource usage.

Para a versão em português vá para: LEIAME.md

Spotlight Dimmer is a program for Windows that dims all the monitors other than the monitor that has the currently focused program.

It's intended to help people who use multiple monitores to focus and assist in quickly noticing which window has the current focus while changing focus with shortcuts like alt + tab. It's specially useful for users who navigate mainly with the keyboard. It helps to avoid silly situations like typing terminal commands on Teams because you're looking at a screen while the focus is on the other screen.

Features

• Ultra-lightweight: Only ~7.6 MB RAM usage, ~561 KB binary size
• Native Windows API: No browser engine overhead, instant startup
• Perfect transparency: Smooth 50% dimming (customizable) on inactive displays
• Click-through overlays: Overlays don't interfere with mouse/keyboard input
• Automatic focus tracking: Detects active window and display changes in real-time (100ms polling)
• Display hotplug support: Automatically recreates overlays when displays are connected/disconnected
• Persistent configuration: Settings saved in TOML format at %APPDATA%\spotlight-dimmer\config.toml
• CLI configuration tool: Manage settings without running the main application

Installation

Option 1: Install via npm (Recommended)

The easiest way to install Spotlight Dimmer is through npm:

npm install -g spotlight-dimmer

Requirements:

• Node.js 14 or higher
• Windows x64

The package includes pre-built binaries - no compilation needed! After installation, the commands spotlight-dimmer and spotlight-dimmer-config will be available globally.

Uninstall:

npm uninstall -g spotlight-dimmer

Option 2: Build from Source

If you prefer to build and install manually using Cargo:

cargo build --release --bin spotlight-dimmer --bin spotlight-dimmer-config

Binaries will be in target\release\:

• spotlight-dimmer.exe - Main application
• spotlight-dimmer-config.exe - Configuration tool

You can also use the PowerShell installation script:

.\install.ps1

Usage

Running the Application

Simply run spotlight-dimmer.exe:

spotlight-dimmer.exe

The application will:

1. Load configuration from %APPDATA%\spotlight-dimmer\config.toml (or create default)
2. Detect all connected displays
3. Create semi-transparent overlay windows on each display
4. Monitor active window focus and hide overlay on active display
5. Run indefinitely until terminated

Stopping the Application

To stop the running application, use PowerShell:

Get-Process spotlight-dimmer | Stop-Process

Or use Task Manager to end the spotlight-dimmer.exe process.

Configuration Tool

Use spotlight-dimmer-config.exe to manage settings:

# Show current configuration
spotlight-dimmer-config status

# Enable/disable dimming
spotlight-dimmer-config enable
spotlight-dimmer-config disable

# Set overlay color (RGB 0-255, alpha 0.0-1.0)

spotlight-dimmer-config color 0 0 0 0.7      # 70% black overlay
spotlight-dimmer-config color 50 50 50 0.3   # 30% gray overlay

# Reset to defaults
spotlight-dimmer-config reset

Note: Configuration changes are automatically detected and reloaded within 2 seconds. No restart needed!

Configuration File

Configuration is stored at %APPDATA%\spotlight-dimmer\config.toml:

is_dimming_enabled = true



[overlay_color]

r = 0

g = 0

b = 0

a = 0.5

Architecture

Core Application (spotlight-dimmer.exe)

• Memory usage: ~7.6 MB
• Binary size: 561 KB
• Implementation: Pure Windows API with Rust winapi crate
• Overlay technology: Layered windows (WS_EX_LAYERED) with alpha blending
• Focus monitoring: 100ms polling using GetForegroundWindow() and MonitorFromWindow()

Configuration Tool (spotlight-dimmer-config.exe)

• Binary size: 627 KB
• Implementation: CLI tool using clap for argument parsing
• Configuration: TOML format via toml crate

Key Technical Details

• Click-through: WS_EX_TRANSPARENT flag ensures overlays don't capture input
• Always on top: WS_EX_TOPMOST keeps overlays above other windows
• No taskbar: WS_EX_TOOLWINDOW prevents overlays from appearing in Alt+Tab
• No focus: WS_EX_NOACTIVATE prevents overlays from stealing focus
• Transparency: SetLayeredWindowAttributes() with LWA_ALPHA for smooth dimming

Comparison with Tauri Version

Development

Project Structure

.
├── src/
│   ├── main_new.rs          # Main application entry point
│   ├── config_cli.rs        # Configuration CLI tool
│   ├── config.rs            # Configuration system (TOML)
│   ├── overlay.rs           # WinAPI overlay implementation
│   └── platform/
│       ├── mod.rs           # Cross-platform traits
│       └── windows.rs       # Windows display/window management
├── Cargo.toml               # Rust dependencies
└── target/release/          # Build output

Building

cargo build --release

Dependencies

• serde - Configuration serialization
• toml - TOML configuration parsing
• winapi - Windows API bindings

Known Limitations

Window Dragging Behavior

When dragging windows between monitors with the mouse, overlays are temporarily hidden to prevent system instability:

• During drag: All overlays disappear when the left mouse button is pressed
• After drag: Overlays reappear with correct visibility when the mouse button is released
• Why: Windows' drag-and-drop message loop conflicts with overlay visibility updates, causing system instability if overlays remain visible
• Workaround: Use keyboard shortcuts (Win+Arrow keys) for instant overlay updates without hiding

This is a Windows API limitation, not a bug. Focus changes and keyboard-based window movement work instantly without hiding overlays.

Roadmap

• System tray icon (optional, using trayicon crate)
• Hot reload configuration without restart (2-second detection window)
• Per-display color customization
• Linux support (using X11/Wayland)

License

MIT

Credits

Built with Rust and the Windows API for maximum performance.