# System76 Launch 3 Custom Firmware

Custom QMK firmware for the [System76 Launch 3](https://system76.com/accessories/launch) keyboard, built with Nix.

## Features

### OS Mode

The keyboard auto-detects the host OS on plug-in and rewrites shortcuts so that muscle memory from Linux (Ctrl-based shortcuts) works on macOS. Mode can also be cycled manually with **FN+Del**.

Four modes, indicated by RGB flash color:

| Mode    | Color  | Behavior |
|---------|--------|----------|
| None    | Red    | No rewrites, all keys pass through |
| Linux   | Green  | Super+Up → Super+W (KDE overview) |
| Mac     | Yellow | Full Ctrl→Cmd rewrite (see table below) |
| Windows | Blue   | Super+Up → Super+Tab (Task View) |

#### Mac Mode Shortcut Rewrites

| Input (Linux muscle memory) | Output (macOS) | Action |
|---|---|---|
| Ctrl+C/V | Cmd+C/V | Copy/Paste |
| Ctrl+Shift+C/V | Cmd+C/V | Terminal copy/paste |
| Ctrl+X/A/S/F | Cmd+X/A/S/F | Cut, Select All, Save, Find |
| Ctrl+W/L/R | Cmd+W/L/R | Close tab, Address bar, Reload |
| Ctrl+Z/T/N | Cmd+Z/T/N | Undo, New tab, New window |
| Ctrl+Shift+Z/T/N | Cmd+Shift+Z/T/N | Redo, Reopen tab, Incognito |
| Ctrl+Left/Right | Alt+Left/Right | Word navigation |
| Ctrl+Alt+Left/Right | Ctrl+Left/Right | Switch workspace |
| Ctrl+Backspace | Alt+Backspace | Delete word |
| Home/End | Cmd+Left/Right | Line start/end |
| Alt+F4 | Cmd+Q | Quit app |
| Alt+Tab | Cmd+Tab | App switcher |
| Super tap | Cmd+Space | Spotlight |
| Super+Up | Ctrl+Up | Mission Control |

#### Layer Override

When any modifier is held (in any mode except None), the FN layer is bypassed — keys always resolve from Layer 0. This allows bypassing shortcut rewrites when wanted.  For example, if you want to send Ctrl+c on mac (which is normally rewritten to cmd+c), simply use fn+Ctrl+c instead.

### Layer 1 (FN Layer)

| Key | FN+ |
|-----|-----|
| ESC | Print Screen |
| F1/F2/F3 | Mute / Vol Down / Vol Up |
| 1/2/3 | Play-Pause / Prev / Next |
| 0/-/= | LED Toggle / LED Down / LED Up |
| Del | Cycle OS Mode |
| Arrows | Home / PgDn / PgUp / End |

Modifier keys (Shift, Ctrl, GUI, Alt) cannot be rebound on Layer 1 due to the layer override behavior.

## Building

Requires [Nix](https://nixos.org/) with flakes enabled.

```sh
nix build
```

The `.uf2` firmware file is output to `./result/`.

### Flashing

1. Put the keyboard into bootloader mode (press the reset button on the bottom)
2. Copy the `.uf2` file to the mounted USB drive

### Development Shell

```sh
nix develop
qmk compile -kb system76/launch_3 -km custom
```

## Project Structure

```
keymaps/custom/
├── keymap.c       # Keymaps, shortcut rewrites, layer override
├── os_detect.c    # OS auto-detection, RGB flash indicator
├── os_mode.h      # OS mode enum and shared state
└── rules.mk       # QMK build flags
qmk_firmware/      # System76 QMK fork (git submodule)
flake.nix          # Nix build system
```