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README.md

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+# 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
+```