Cycle the Windows default audio output from a global hotkey

A single 306 KB GUI-subsystem binary with no tray icon and no settings
window: one hidden popup owns the hotkey, the device-change callbacks and
the flyout, and the process sleeps in GetMessageW at 0% CPU in between.

Windows exposes no public API for setting the default audio endpoint, so
this uses the undocumented IPolicyConfig COM interface, confined to
src/policy.rs. Verified working on build 26200. Everything else -
enumeration, IMMNotificationClient, RegisterHotKey, the flyout - is
documented Win32.

The flyout uses the DWM acrylic backdrop and rounded corners the shell
uses for its own flyouts, with text drawn via DrawThemeTextEx and
DTT_COMPOSITED into a 32-bpp DIB, which is what keeps GDI from leaving
black fringes on a composited surface.

Console and Multimedia roles move by default; Communications stays put so
voice chat keeps its headset. Config is a hand-parsed INI reloaded on the
next keypress, keeping the dependency list to the windows crate alone.

Built by cross-compiling from a devcontainer with mingw-w64, so nothing is
installed on the Windows host and the exe imports only system DLLs.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

README.md

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+# win-audio-output-switcher
+
+A global hotkey that cycles the Windows 11 default audio output, and a flyout that tells
+you where you landed.
+
+No tray icon, no window, no settings dialog. One 300 KB executable that sits in
+`GetMessageW` at 0% CPU until you press the key.
+
+```
+Ctrl+Alt+A  ->  Casque (FiiO USB DAC)
+Ctrl+Alt+A  ->  Haut-parleurs (Focusrite USB Audio)
+Ctrl+Alt+A  ->  Haut-parleurs (Steam Streaming Speakers)
+```
+
+## Behaviour
+
+- **One hotkey, forward through a list.** Configure which outputs are in the cycle and in
+  what order, or leave it unconfigured and cycle everything that is plugged in.
+- **Console and Multimedia roles only.** Windows keeps three separate defaults; the
+  Communications one is left alone by default, so a headset stays pinned for voice chat
+  while media follows the hotkey. Change it with `roles =` if you disagree.
+- **Stays in sync.** Switch outputs from the Windows volume flyout, or unplug a DAC, and
+  the next press continues from what Windows actually has selected.
+- **Never steals focus.** The flyout is click-through and non-activating, so pressing the
+  hotkey mid-game or mid-sentence does not pull you out of what you were doing.
+
+## Build
+
+Everything happens in the devcontainer — nothing is installed on the Windows side, and the
+resulting binary imports only system DLLs.
+
+```bash
+cargo build --release
+# -> target/x86_64-pc-windows-gnu/release/audioswitch.exe
+```
+
+The container cross-compiles with mingw-w64 but cannot launch a Windows process, so
+deploying is a host-side step. From a **WSL shell outside the container**:
+
+```bash
+scripts/deploy.sh      # copies to %LOCALAPPDATA%\AudioSwitch and (re)starts it
+```
+
+## Use
+
+`deploy.sh` puts the binary at `%LOCALAPPDATA%\AudioSwitch\audioswitch.exe`. It is **not
+on `PATH`**, so out of the box every invocation is a full path. In PowerShell:
+
+```powershell
+& "$env:LOCALAPPDATA\AudioSwitch\audioswitch.exe"            # run in the background
+& "$env:LOCALAPPDATA\AudioSwitch\audioswitch.exe" --list     # and so on
+```
+
+or from `cmd.exe`:
+
+```bat
+"%LOCALAPPDATA%\AudioSwitch\audioswitch.exe" --list
+```
+
+That gets old fast. Pick one of these once and the short commands below work as written:
+
+```powershell
+# A profile function - run `notepad $PROFILE` and add:
+function audioswitch { & "$env:LOCALAPPDATA\AudioSwitch\audioswitch.exe" @args }
+```
+
+```powershell
+# ...or put the folder on your user PATH (affects new terminals only):
+$old = [Environment]::GetEnvironmentVariable('Path', 'User')
+[Environment]::SetEnvironmentVariable('Path', "$old;$env:LOCALAPPDATA\AudioSwitch", 'User')
+```
+
+### Verbs
+
+```
+audioswitch                  Run in the background (single instance)
+audioswitch --list           List outputs and their endpoint IDs
+audioswitch --config         Create/open the config file
+audioswitch --quit           Stop the running instance
+audioswitch --autostart on   Start with Windows (off to undo)
+```
+
+`--autostart` records the full path of the exe as it was when you ran it, so it does not
+care about `PATH` either way.
+
+The verbs that print borrow the terminal they were launched from. Because this is a
+GUI-subsystem binary, a shell may hand back the prompt before the output arrives; pipe it
+(`audioswitch --list | Out-String`) if the interleaving bothers you.
+
+## Config
+
+`%APPDATA%\win-audio-output-switcher\config.ini`, created by `--config` already populated
+with the outputs you have right now. Edits apply on the next hotkey press — no restart.
+
+```ini
+hotkey = ctrl+alt+a
+roles = console,multimedia
+osd_position = bottom-center     ; top-center, center, bottom-right, top-right
+osd_duration_ms = 1400
+osd_fade = false
+
+; Cycle order. Comment out a line to drop that output.
+; Delete the whole section to cycle every active output instead.
+[devices]
+{0.0.0.00000000}.{a51fdbbf-...} = Casque (FiiO USB DAC-E07K)
+{0.0.0.00000000}.{ff879682-...} = Haut-parleurs (Focusrite USB Audio)
+```
+
+`osd_fade` is off by default. Fading needs `WS_EX_LAYERED`, which on some builds suppresses
+the acrylic backdrop that makes the flyout look like part of the shell. Turn it on and see
+which you prefer on your machine.
+
+## Known limitations
+
+- **Elevated windows swallow the hotkey.** `RegisterHotKey` does not fire while a window
+  running as administrator has focus, unless this app is elevated too. Inherent to the API.
+- **WASAPI exclusive-mode apps do not follow.** Some DAWs and games hold an endpoint open
+  exclusively and keep playing to the old device until restarted.
+- **Exclusive-fullscreen games hide the flyout.** The switch still happens; a topmost
+  window simply cannot draw over an exclusive-fullscreen swap chain.
+
+## The undocumented bit
+
+Windows exposes no public API for setting the default audio endpoint. Like every other
+tool in this space, this one uses the undocumented `IPolicyConfig` COM interface
+(`CLSID {870af99c-…}`, `IID {f8679f50-…}`). It has worked from Vista through current
+Windows 11 builds, but it is not contractual.
+
+It is confined to [`src/policy.rs`](src/policy.rs) — about forty lines, all of it the
+vtable declaration. Everything else (enumeration, change notifications, the hotkey, the
+flyout) is documented Win32. If a Windows update ever breaks this tool, that is the file
+to look at first.