Anchor

A headless macOS daemon that keeps the Dock anchored to a specific display in multi-monitor setups.

By default, macOS lets the Dock migrate to whichever display the cursor approaches — anchor stops that. It intercepts mouse movements near the Dock trigger zones on non-anchor displays and, if the Dock ever drifts, nudges it back automatically at startup.

Requirements

• macOS (tested on macOS Sequoia)
• Swift compiler (swiftc) — included with Xcode Command Line Tools

Important

The binary must be granted Accessibility access before it can intercept mouse events: System Settings → Privacy & Security → Accessibility

Installation

1. Compile the binary

swiftc -framework Cocoa -framework ApplicationServices anchor.swift -o anchor

2. Move it somewhere on your PATH (the default plist expects /usr/local/bin/anchor)

sudo mv anchor /usr/local/bin/anchor

3. Install the LaunchAgent to run anchor automatically at login

cp flesch.anchor.plist ~/Library/LaunchAgents/
launchctl load ~/Library/LaunchAgents/flesch.anchor.plist

Usage

anchor [display_index]   # 0 = primary display (default), 1 = second display, etc.
anchor --list            # print available displays and exit

Examples

# Anchor the Dock to the primary display (default)
anchor

# Anchor the Dock to the second display
anchor 1

# List available displays
anchor --list
# [0] Built-in Display (primary) — (0.0, 0.0, 1512.0, 982.0)
# [1] Pro Display XDR — (1512.0, 0.0, 3008.0, 1692.0)

How it works

On startup, anchor:

1. Enumerates active displays and selects the anchor display by index.
2. Detects where the Dock currently lives using the Accessibility API.
3. If the Dock is on the wrong display, it synthesizes a smooth mouse movement to the anchor display's Dock trigger zone to relocate it.
4. Installs a low-level CGEventTap that blocks mouse movements from entering the Dock trigger zone on any non-anchor display, preventing macOS from moving the Dock away.

Synthetic events are tagged with a private marker so anchor can distinguish its own events from real user input and never accidentally blocks itself.

Configuration

The flesch.anchor.plist LaunchAgent can be customized:

Key	Default	Description
ProgramArguments	/usr/local/bin/anchor	Path to the binary. Add a display index as a second argument to pin to a non-primary display.
KeepAlive	true	Restart automatically if the daemon crashes.
StandardOutPath	/tmp/anchor.log	Stdout log file.
StandardErrorPath	/tmp/anchor.err	Stderr log file.

To reload after changes:

launchctl unload ~/Library/LaunchAgents/flesch.anchor.plist
launchctl load   ~/Library/LaunchAgents/flesch.anchor.plist

Troubleshooting

The Dock still moves to another display
Make sure the binary has been granted Accessibility access. anchor will print a warning and exit if the permission is missing.

Failed to create event tap
Revoke and re-grant Accessibility access in System Settings, then restart anchor.

Checking logs

tail -f /tmp/anchor.log
tail -f /tmp/anchor.err