ScrollSense

Intelligent Natural Scroll Switching for macOS

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Overview

ScrollSense is a lightweight macOS daemon that automatically switches the system's Natural Scrolling behavior based on the active input device — mouse or trackpad.

macOS uses a single global setting for natural scrolling. However, many users prefer:

• ✅ Natural scrolling ON for trackpad
• ❌ Natural scrolling OFF for mouse

ScrollSense intelligently detects which device is currently being used and dynamically updates the system preference — giving users the correct scrolling behavior automatically.

No manual toggling. No friction. No System Settings visits.

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The Problem

macOS treats scrolling direction as a global setting:

System Settings → Trackpad / Mouse → Natural Scrolling

But in reality:

• Trackpad scrolling feels natural when enabled
• Mouse wheel scrolling feels inverted when enabled
• Switching between devices requires manual toggling
• This becomes extremely frustrating for developers and power users

There is no native per-device solution.

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The Solution

ScrollSense runs as a background daemon that:

1. Listens to low-level input events (scroll wheel) via CGEventTap
2. Detects whether the event originated from an external mouse or trackpad
3. Compares the detected device with user-defined preferences
4. Updates macOS natural scrolling only if needed
5. Avoids redundant system calls through internal state tracking

It simulates per-device scroll preferences — even though macOS does not support it natively.

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Installation

Build from Source

# Clone the repository
git clone https://github.com/your-username/ScrollSense.git
cd ScrollSense

# Build release binary
swift build -c release

# Copy to /usr/local/bin (optional)
cp .build/release/scrollSense /usr/local/bin/

Permissions Required

ScrollSense requires Accessibility permission to monitor input events:

1. Open System Settings → Privacy & Security → Accessibility
2. Add your terminal app (e.g., Terminal, iTerm2) or the scrollSense binary
3. Toggle the permission ON

This is required only once during setup.

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Usage

Set Preferences

Define your preferred scroll behavior per device:

scrollSense set --mouse false --trackpad true

This saves to ~/.scrollsense.json:

{
  "mouseNatural" : false,
  "trackpadNatural" : true
}

Start Daemon (Foreground / Debug Mode)

scrollSense run --debug

Prints real-time debug output:

[scrollSense] scrollSense daemon starting...
[scrollSense DEBUG 2024-01-15T10:30:00Z] Initial system natural scroll: true
[scrollSense DEBUG 2024-01-15T10:30:00Z] Config: mouse=false, trackpad=true
[scrollSense] scrollSense daemon running. Listening for scroll events...
[scrollSense] Debug mode enabled. Press Ctrl+C to stop.
[scrollSense DEBUG 2024-01-15T10:30:05Z] Device switch: none → mouse
[scrollSense DEBUG 2024-01-15T10:30:05Z] Applying scroll change: natural=false (for Mouse)

Start Daemon (Background)

scrollSense start

This launches the daemon as a background process and tracks it via a PID file (/tmp/scrollsense.pid).

Stop Daemon

scrollSense stop

Sends SIGTERM to the running daemon and cleans up the PID file.

Run Daemon (Foreground)

For manual/interactive use without backgrounding:

scrollSense run

Or install as a LaunchAgent for auto-start at login:

scrollSense install

Check Status

scrollSense status

Output:

  scrollSense Status
  ──────────────────────────────────
  Daemon: Running (PID: 12345)
  Mouse natural scroll: OFF
  Trackpad natural scroll: ON
  System natural scroll: OFF
  Config file: /Users/you/.scrollsense.json
  LaunchAgent installed: No
  ──────────────────────────────────

Install LaunchAgent (Auto-Start at Login)

scrollSense install

To specify a custom binary path:

scrollSense install --path /usr/local/bin/scrollSense

Uninstall LaunchAgent

scrollSense uninstall

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Architecture

ScrollSense is built natively using Swift and macOS system frameworks.

Project Structure

Sources/
├── ScrollSense/              # Core library (ScrollSenseCore)
│   ├── Models.swift          # InputDevice, ScrollPreferences, DaemonState
│   ├── ConfigManager.swift   # Preferences storage (~/.scrollsense.json)
│   ├── DeviceDetector.swift  # CGEvent-based device detection
│   ├── ScrollController.swift # System scroll setting read/write
│   ├── StateManager.swift    # Runtime state & optimization
│   ├── ScrollDaemon.swift    # Main event loop & switching logic
│   ├── PIDManager.swift      # PID file tracking for daemon state
│   ├── LaunchAgentManager.swift # LaunchAgent install/uninstall
│   ├── Logger.swift          # Logging utility
│   └── ScrollSense.swift     # CLI command definitions
├── ScrollSenseApp/
│   └── main.swift            # Executable entry point
Tests/
└── ScrollSenseTests/
    └── ScrollSenseTests.swift # Unit tests (Swift Testing)

Module Responsibilities

Module	Responsibility
Models	Data types: InputDevice, ScrollPreferences, DaemonState
ConfigManager	Load/save preferences from ~/.scrollsense.json
DeviceDetector	Detect mouse vs trackpad from CGEvent fields
ScrollController	Read/write macOS com.apple.swipescrolldirection via CoreFoundation CFPreferences API
StateManager	Track runtime state, optimize by avoiding redundant writes
ScrollDaemon	Main event tap loop, orchestrates detection → comparison → update
PIDManager	PID file tracking (/tmp/scrollsense.pid) for daemon lifecycle
LaunchAgentManager	Install/uninstall macOS LaunchAgent for auto-start
Logger	Structured logging with debug/info/warning/error levels

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Optimized Runtime Logic

On daemon start:
  → Load config
  → Read current system scroll state
  → Wait for first input event

On scroll event:
  If desired_setting == last_applied_setting
    → Do nothing (skip)
  Else
    → Update macOS scroll direction
    → Record new applied state

On device switch:
  → Log the switch (debug mode)
  → Evaluate if scroll change is needed

This ensures:

• ✅ No repeated system writes
• ✅ No unnecessary preference API calls
• ✅ Setting changes dispatched asynchronously — zero scroll lag
• ✅ No system-wide side effects (no process kills)

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Device Detection

ScrollSense uses the CGEvent field .scrollWheelEventIsContinuous to distinguish devices:

Value	Device	Description
1	Trackpad	Continuous/momentum scrolling
0	Mouse	Discrete scroll wheel steps

Additional fields available for debug inspection:

• scrollWheelEventMomentumPhase
• scrollWheelEventScrollPhase
• scrollWheelEventDeltaAxis1 (vertical)
• scrollWheelEventDeltaAxis2 (horizontal)

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Example Scenario

User preference: Mouse → Natural OFF, Trackpad → Natural ON

Step	Action	Result
1	User scrolling with trackpad	Natural ON (already set)
2	User grabs mouse, scrolls	ScrollSense detects mouse → Natural OFF applied
3	User continues using mouse	No checks performed (optimized)
4	User touches trackpad	Device switch detected → Natural ON applied

Seamless. Invisible. Instant.

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Technical Stack

Component	Technology
Language	Swift 5.9+
Event Monitoring	CoreGraphics (CGEventTap)
System Preferences	CoreFoundation CFPreferences API (com.apple.swipescrolldirection)
CLI Framework	Swift Argument Parser
Build System	Swift Package Manager
Auto-Start	macOS LaunchAgent
Testing	Swift Testing framework

No Electron. No UI frameworks. Pure native macOS.

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Command Reference

Command	Description
scrollSense start	Start daemon in the background
scrollSense stop	Stop the running daemon
scrollSense run	Run daemon in the foreground
scrollSense run --debug	Run daemon with verbose debug logging
scrollSense set --mouse <bool> --trackpad <bool>	Set per-device preferences
scrollSense status	Show current preferences, daemon state, and system state
scrollSense install	Install LaunchAgent for auto-start at login
scrollSense install --path <path>	Install with custom binary path
scrollSense uninstall	Remove LaunchAgent
scrollSense --version	Show version
scrollSense --help	Show help

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Configuration

Preferences are stored in ~/.scrollsense.json:

{
  "mouseNatural" : false,
  "trackpadNatural" : true
}

The daemon reloads this file every 2 seconds, so changes made via scrollSense set are picked up automatically without restarting.

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Limitations

• macOS has only one global natural scroll setting
• ScrollSense dynamically switches it — it cannot set per-device simultaneously
• Requires Accessibility permission
• Requires macOS 12.0 (Monterey) or later

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Future Enhancements

• Menu bar app
• Device-specific sensitivity profiles
• GUI preference panel
• Homebrew distribution
• Notarized binary
• Strict mode (periodic system preference verification)
• Per-device custom scroll speed
• Scroll usage statistics

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Target Users

• Developers
• Designers
• MacBook users with external mouse
• Power users
• Anyone switching between devices daily

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Vision

ScrollSense aims to feel like a native macOS behavior enhancement.

Invisible. Instant. Reliable.

It removes friction from daily workflow by intelligently adapting to the user's current input device.

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License

MIT

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One-Line Pitch

ScrollSense is a native macOS daemon that automatically switches scroll direction based on whether you're using a mouse or trackpad.