fix(macos): track Fn flag bit in FlagsChanged handler

[that-yolanda]

Problem

On macOS the Fn key is completely unobservable — pressing it produces no KeyDown/KeyUp events, even though Key::Function exists and kVK_Function (keycode 63) is already mapped in the keycode table.

Root cause

Key::Function is mapped in the keycode table (63 => Key::Function), but flag_bit_for_key() in src/platform/macos/tap.rs has no matching arm. So every Fn FlagsChanged event falls through to the _ => 0 fallback and is silently dropped:

fn flag_bit_for_key(key: Key) -> u64 {
    match key {
        // ...
        Key::CapsLock => CGEVENT_FLAG_CAPS_LOCK,
        _ => 0,   // ← Key::Function lands here → event dropped
    }
}

Fix

Map Key::Function to kCGEventFlagMaskSecondaryFn (0x800000), mirroring how the other modifiers are tracked:

const CGEVENT_FLAG_FUNCTION: u64 = 0x800000; // kCGEventFlagMaskSecondaryFn

fn flag_bit_for_key(key: Key) -> u64 {
    match key {
        // ...
        Key::CapsLock => CGEVENT_FLAG_CAPS_LOCK,
        Key::Function => CGEVENT_FLAG_FUNCTION,   // ← added
        _ => 0,
    }
}

Verification

Verified locally on macOS: pressing Fn now emits KeyDown(Function) / KeyUp(Function) events (previously nothing was emitted). Other modifiers are unaffected.

Summary by CodeRabbit

• Bug Fixes
  • Fixed handling of the Function (Fn) modifier on macOS, enabling correct press and release detection for Function key events.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro

Run ID: a956ba1f-07c9-4d58-aeb1-88b2c3cebc0f

📥 Commits

Reviewing files that changed from the base of the PR and between a1b3385 and 6fecb8b.

📒 Files selected for processing (1)

• src/platform/macos/tap.rs

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📝 Walkthrough

Walkthrough

Adds support for the macOS Fn (Function) modifier key in src/platform/macos/tap.rs. A new CGEVENT_FLAG_FUNCTION bit constant is defined, and flag_bit_for_key is extended to map Key::Function to that constant, enabling correct press/release edge detection for the Fn modifier.

Changes

Fn Modifier Recognition

Layer / File(s)	Summary
Fn flag constant and key mapping src/platform/macos/tap.rs	Defines CGEVENT_FLAG_FUNCTION as the CGEvent bit for the Fn modifier and adds Key::Function => CGEVENT_FLAG_FUNCTION to flag_bit_for_key, so FlagsChanged processing can classify Fn as a known modifier and emit synthesized key events.

Estimated code review effort

🎯 1 (Trivial) | ⏱️ ~2 minutes

Poem

🐇 Hop hop, a tiny fix today,
The Fn key was lost, now found its way!
A constant defined, a mapping laid bare,
No more unknown modifier floating in air.
Two lines of Rust, the rabbit declares — done! 🎉

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly and accurately describes the main change: adding Fn flag bit tracking in the FlagsChanged handler for macOS, which is the core fix in this pull request.
Docstring Coverage	✅ Passed	Docstring coverage is 100.00% which is sufficient. The required threshold is 80.00%.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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