Capswitch Design

Capswitch is a small native macOS menu-bar application for a user who wants physical Caps Lock to cycle keyboard layouts without the stock input-source shortcut HUD or animation.

Intent

On a plain Caps gesture, suppress the system Caps Lock event and switch to the next enabled keyboard input source on Caps release. Reserve Caps+Option for the real Caps Lock behavior.

Goals

Global Caps interception through Accessibility-authorized CGEventTap.
Plain Caps switches once on release.
Caps+Option toggles real Caps Lock in either key order.
Input sources are selected directly through Carbon TIS APIs.
Menu-bar indicator shows the current input source and permission state.
Build and bundle from Taskfile/CLI on macOS without an Xcode project.

Non-goals

Replacing or modifying macOS input-source configuration.
Supporting App Store sandbox or notarized distribution in the MVP.
Implementing a full preferences window.
Supporting arbitrary remapping beyond Caps and Caps+Option.

Interaction Contract

The event tap suppresses Caps down/up events. It records whether Option was already down or became active while Caps is held. On Caps release, no Option seen selects the next source; any Option seen schedules a synthetic clean Caps down/up. If Option remains held, the synthetic Caps event is posted when Option is released.

Platform Contract

Swift 5-compatible source targeting macOS 14.0+.
AppKit status item, CoreGraphics event tap, ApplicationServices Accessibility check, Carbon TIS input source APIs.
Stable bundle identifier `dev.local.Capswitch`.
`LSUIElement=true` so the app has no Dock icon.

Verification

Linux may run only platform-independent/static checks because the remote host has no Swift/macOS SDK. The authoritative build and XCTest run are on macOS 14.8.9 with Command Line Tools installed.

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