CoopAllTheThings/README.md

# CoopAllTheThings

Steam **Remote Play Together (RPT)** for any XInput game — without breaking DRM,
achievements, or playtime.

Existing "donor game" tools (e.g. RemotePlayWhatever) copy a target game's files
into a donor game's folder and rename the executable so Steam streams the target
under the donor's appid. That breaks DRM-protected games, breaks achievements,
and credits playtime to the donor.

CoopAllTheThings takes a different approach: the **real game runs normally under
its own appid** (so DRM, achievements, and playtime all work), while a lightweight
**mirror app runs under the donor appid**. The mirror presents a borderless
window that is a live copy of the game's video + audio, and forwards the guests'
input back into the real game. Steam's RPT captures the mirror window — so any
XInput game becomes Remote-Play-Together-able.

The end-to-end path is working: launched under a donor appid, the host streams a
live video + audio mirror of a separately-running game over Remote Play Together
and forwards guest controllers back into it.

## Architecture

| Concern | Mechanism | Component |
| --- | --- | --- |
| Receive guest input | XInput (RPT delivers guest pads to the focused window); optional, opt-in Steam Input when built with the Steamworks SDK | `coop_host.exe` |
| Forward input to game | DLL injection + XInput hook (SafetyHook) — game sees *only* our pad | `coop_hook.dll` |
| Forward mouse + keyboard | Opt-in MKB subsystem: host streams its window's clicks/keys, the hook posts the matching window messages and synthesizes `GetAsyncKeyState`/`GetKeyboardState`/`GetCursorPos` for polling games | `coop_hook.dll` + `coop_host.exe` |
| Keep game running unfocused | Hook spoofs focus so the game polls while the host holds OS focus | `coop_hook.dll` |
| Mirror video (default) | Windows Graphics Capture of the game window, letterboxed into the host window | `coop_host.exe` |
| Mirror video (hooked) | Injected Present / OpenGL hook copies the backbuffer into a shared keyed-mutex texture the host samples (lower latency, no capture border) | `coop_hook.dll` + `coop_host.exe` |
| Mirror audio | Injected render-hook copies each of the game's WASAPI render streams into its own shared ring and silences the game locally (no echo); the host mixes the streams (soft-clipped); WASAPI process loopback is the automatic fallback | `coop_hook.dll` + `coop_host.exe` |
| Host ↔ hook IPC | Named shared memory (seqlock for input, status back-channel, video/audio/log shares) | `common/` |

The hooked video path has three producers: **Direct3D (DXGI)** hooks
`IDXGISwapChain::Present` / `Present1` and copies the backbuffer — directly for
D3D11 games (the backbuffer is an `ID3D11Texture2D`), via a **D3D11On12
bridge** for D3D12 games (wrap the `ID3D12Resource` backbuffer, `CopyResource` into
the shared texture), and via a **D3D10 read-back** for D3D10 games (their backbuffer's
D3D11 view is empty and a feature-level-10 device can't host the shared texture, so
read it through the game's own D3D10 device and upload it via a hook-owned D3D11
device); **Direct3D 9** inline-hooks `IDirect3DDevice9::Present` (vtable index 17) and
read-backs the backbuffer with `GetRenderTargetData` (a D3D9 surface isn't D3D11-shareable),
swizzling BGRA→RGBA and uploading via a hook-owned D3D11 device — one path covers both plain
D3D9 and D3D9Ex; **OpenGL** hooks `SwapBuffers` / `wglSwapBuffers` and reads the
backbuffer with `glReadPixels` (for games that never touch DXGI, e.g. Phantom
Brave); and **Vulkan** inline-hooks the `vulkan-1.dll` `vkGetInstanceProcAddr` export to
intercept the resolution chain (`vkCreateInstance` / `vkCreateDevice` /
`vkCreateSwapchainKHR` / `vkQueuePresentKHR`) and reads the presented image back with
`vkCmdCopyImageToBuffer` — but only when the hook is present **before** the game initializes
Vulkan (it caches its present pointer at init), so the Vulkan path needs early presence via
Auto-attach or the opt-in implicit **capture layer** (`coop_vk_layer`, registered per-user by the
Injection panel's "Set up Vulkan layer" checkbox and scoped to the target game); a too-late attach
shows a red relaunch banner and falls back to WGC. The host samples the copy as plain UNORM
(`srgb_to_unorm`) so `*_SRGB`-backbuffer games mirror at correct brightness. **WGC remains the
default** and covers anything the hooked path doesn't.

## Limitations

- **Anti-cheat:** the input path injects `coop_hook.dll` into the target game.
  Games protected by kernel-level anti-cheat (Easy Anti-Cheat, BattlEye,
  Vanguard, etc.) will detect the injected module and may **kick the player or
  issue a ban**. Such games are explicitly **out of scope and unsupported** — do
  not use CoopAllTheThings with them. The tool targets single-player and
  co-op/local-multiplayer titles without active anti-cheat.
- **XInput only:** the game must read controllers via XInput (the common case).
  DirectInput-only / RawInput-only games are not handled.
- **32-bit games supported via a helper:** the host is x64, but the build also
  produces an x86 hook DLL (`coop_hook_x86.dll`) and a 32-bit injector helper
  (`coop_inject_x86.exe`). When the target is a 32-bit (WOW64) process the host
  detects it (`IsWow64Process2`) and shells out to the helper to load the x86 DLL
  (a 64-bit process can't cleanly inject a 32-bit one). The shared-memory IPC is
  fixed-width / bitness-stable, so the x64 host and x86 hook interoperate.
- **Local audio echo on the fallback path:** when the render-hook is active it
  silences the game's local playback while mirroring it, so there is no echo. If
  the hook can't attach or the game uses an unhooked render path, the host falls
  back to process-loopback capture, which does *not* mute the game — so the local
  machine hears the audio twice (guests hear it once). The Audio panel shows which
  path is active.
- **Hooked audio can only recover a pre-existing stream's *sample rate*, not its
  channels/bit-depth.** The tool injects into an already-running game, so the audio
  render-hook usually never saw the game's `IAudioClient::Initialize`. It recovers the
  true **sample rate** by measuring the render cadence (so playback pitch is correct,
  e.g. Godot/Brotato's 44100 Hz on a 48000 Hz endpoint), but **channels and bit-depth
  can't be detected** — with `AUTOCONVERTPCM` `GetBuffer` returns a fixed staging
  buffer (no buffer stride to measure) and WASAPI exposes no API for a pre-existing
  client's format — so they're *assumed* to match the device mix format. That's correct
  for the common case (engines render stereo float, matching the endpoint, differing
  only in rate). A game rendering a *different* channel count or bit depth than the
  device is mirrored with the wrong layout (garbled audio) on the hooked path, but never
  an over-read/crash: the capture copy is clamped to the readable region (`VirtualQuery`),
  and the local **mute** is done with `AUDCLNT_BUFFERFLAGS_SILENT` (which makes WASAPI
  ignore the buffer's contents), so it never *writes* the wrongly-sized buffer either. So
  every captured stream is silenced locally — guessed or exact — and there is no echo on
  the hooked path. The loopback fallback is always format-correct. The Audio panel shows
  each stream's
  format provenance (*known* / *measuring* / *measured rate* / *low-confidence* /
  *override*) so the assumption is visible, and (under Debug details) lets the operator
  **re-measure** the rate or **override** the format when the guess is wrong. Overrides
  are **remembered per game** (and a format caught exactly at `Initialize` is auto-saved
  as that game's override), so a known-bad game is corrected automatically next launch.
  Streams created *after* injection are captured exactly.
- **Debug-oriented UI:** the ImGui overlay is laid out for diagnosing the
  pipeline, not for end use. F1 hides it entirely so the window is a clean mirror
  for RPT; F2 frees the operator cursor; **F10 saves a PNG screenshot** (back buffer,
  written next to the exe) regardless of window focus or occlusion.

## Roadmap

### Future work

- **Mouse + keyboard forwarding for Raw Input / DirectInput games.** The MKB
  subsystem forwards via window messages (`PostMessage`) plus synthesized
  `GetAsyncKeyState` / `GetKeyboardState` / `GetCursorPos`, which covers message-loop
  and polling games. Games that read keyboard/mouse via **Raw Input** (`WM_INPUT` /
  `GetRawInputData`, e.g. Trails through Daybreak) or **DirectInput**
  (`IDirectInputDevice8::GetDeviceState/GetDeviceData`) don't see it. Add hooks for
  those paths to synthesize the forwarded input there too.

## Building

Requirements: Windows 10/11, Visual Studio 2022 (MSVC + C++ workload), CMake ≥ 3.21.

```sh
git clone --recurse-submodules <repo-url>
# or, if already cloned:
git submodule update --init --recursive

cmake -S . -B build -G "Visual Studio 17 2022" -A x64
cmake --build build --config Debug
# output: bin/Debug/coop_host.exe (+ coop_hook.dll, coop_vk_layer.dll + manifest, test exes)
```

The x64 build also drives a nested Win32 sub-build (CMake `ExternalProject`,
configured into `build/x86/`) that produces `coop_hook_x86.dll` and
`coop_inject_x86.exe` for 32-bit games, staged next to the x64 binaries. Disable
it with `-DCOOP_BUILD_X86_HELPER=OFF` if you don't need 32-bit support.

Third-party dependencies (Dear ImGui, SafetyHook, the official Khronos Vulkan-Headers, and the
`zeux/volk` Vulkan meta-loader) are git submodules under `third_party/`. No vcpkg / package
manager is used; CMake fails with a clear `git submodule update --init --recursive` hint if one
is missing (`coop_require_submodule`).

**Steam Input is optional.** It's enabled automatically when the Steamworks SDK is
vendored at `third_party/steamworks_sdk/` (extract the `steamworks_sdk_*.zip`
there). The SDK isn't redistributable, so it's gitignored and never committed; if
it's absent the host builds XInput-only (no other features depend on it). When
present, the build links `steam_api64.lib`, stages `steam_api64.dll` and the
action manifest next to the host, and also builds `coop_steam_input_probe`.

Steam Input is **off by default and XInput is the primary path**: merely
initializing Steam Input activates Steam's in-process XInput interception, which
hides controllers from XInput unless they're bound to our action set for the
running appid. Enable it (Controllers panel → **Use Steam Input**) only once a
controller is bound to Steam Input for the donor appid.

### clangd / IDE setup

The Visual Studio CMake generator does **not** emit `compile_commands.json`, so
clangd has no include paths and reports false errors. Run
[`gen-compile-commands.bat`](gen-compile-commands.bat) once (and after adding
sources or include dirs); it configures a parallel Ninja build in `build-clangd/`
that produces the database, which [`.clangd`](.clangd) points clangd at. clangd's
clang-cl driver resolves the MSVC / Windows SDK system includes on its own.

## Tests

```sh
ctest --test-dir build -C Debug --output-on-failure
```

> **Fix bugs test-first.** Every bug fix must begin with a test that *reproduces* the bug
> and **fails** on the unfixed code — run it, watch it fail, and confirm it fails for the
> right reason. Only then write the fix, and confirm the same test now passes. A fix without
> a first-failing test is not done: the test is what proves the bug existed, that the change
> addresses it, and that it can't silently come back. (The hooked-audio double-play bug is the
> worked example: `audio_hook_test`'s guessed-path mute assertion was added and seen to fail
> before the one-line mute fix landed.)

- **`hook_selftest`** — in-process check of the IPC + XInput hook core (no game,
  no controller needed).
- **`audio_ring_test`** — unit test of the shared audio ring (lock-free SPSC
  push/pop, wrap-around, format handshake, overrun/drop). No device needed.
- **`audio_mix_test`** — unit test of the multi-stream mixer math (decode / sum /
  soft-clip / encode for float32 + int16). No device needed.
- **`rate_estimator_test`** — unit test of the robust sample-rate estimator (the fix for
  the wrong-rate bug). Feeds synthetic, adversarial render cadences and asserts it
  converges to the right standard rate, rejects burst windows (never commits to a wrong
  neighbour, incl. the real 46205 misread), flags a genuinely non-standard rate
  low-confidence instead of spinning, and ignores idle windows. Pure logic, no device.
- **`audio_overrides_test`** — unit test of the per-game audio override store
  (persist/reload, case-insensitive lookup by image name, and the differing-overwrite
  detection that drives the warning). No device.
- **`tone_analysis_test`** — unit test of the audio fidelity analyzer used by
  `coop_audio_validate` (pitch error in cents, SNR/THD, click + dropout detection) and the WAV
  reader/writer. Synthesizes a clean tone, a wrong-rate (pitch-shifted) tone, a tone with
  injected clicks, and one with silence gaps, and asserts each metric matches what was injected
  (e.g. 44100 played as 48000 → +147 cents). Pure header logic, no device.
- **`render_pacer_test`** — unit test of the mirror's render-feed pacing policy
  (`host/src/audio/render_pacer.hpp`). Simulates a producer/consumer device timeline and asserts
  the shipping `RenderPacer` rides producer jitter that makes the old re-prime-on-partial-fill
  policy withhold available data ~168× and drain the buffer to the brink of silence (the
  under-run / "metallic" bug `coop_audio_validate` found). No device.
- **`audio_hook_test`** — in-process self-test of the WASAPI render-hook's **format
  detection**, the part that gets pitch right. Using a shared configurable
  `ToneSource` (the same render helper `coop_tone` uses), it renders tones at a matrix
  of common formats (44100/48000/96000 Hz, mono/stereo/5.1, 16-bit PCM / 32-bit float)
  and asserts the hook reports the right rate/channels/bits + provenance for **both**
  code paths: **see-init** (hooks installed first → exact `Initialize` format) and
  **guess** (render client pre-exists → device-mix guess whose true rate is measured
  from the cadence, the Brotato/Godot case). Also checks the frames reached the ring
  non-silent **and that the hook muted the game's local playback** (the no-echo guarantee)
  for *both* paths — the guessed-path mute assertion is the regression guard for the
  double-audio bug. Skips cleanly with no audio endpoint.
- **`srgb_format_test`** — unit test of the `srgb_to_unorm` mapping the hooked
  video path uses so `*_SRGB`-backbuffer games aren't darkened. No device.
- **`opengl_hook_test`** — in-process self-test of the OpenGL capture path:
  installs the swap hooks, drives a real OpenGL context (clears the backbuffer to a
  known color, calls `SwapBuffers`), and asserts the detour fired, the frame was
  `glReadPixels`'d into the shared texture, and a second device reads the exact
  pixels back by name. Skips cleanly without an OpenGL / D3D11 device.
- **`dx12_present_hook_test`** — in-process self-test of the Present hook's **D3D12**
  path: drives a real D3D12 swapchain through the (shared) `IDXGISwapChain::Present`
  vtable and asserts the D3D11On12 bridge wraps the `ID3D12Resource` backbuffer and
  copies it into the shared texture, then reads the exact rendered color back by
  name. Skips cleanly without a D3D12 device.
- **`present_hook_test`** — in-process self-test of the Present-hook video path:
  installs the hook, drives a real D3D11 swapchain in the same process (clears the
  backbuffer to a known color and calls `Present`), and asserts the detour fired,
  the backbuffer reached the shared keyed-mutex texture, and a second device can
  open it by name and read the exact pixels back. Skips cleanly if the machine has
  no D3D11 device.
- **`audio_loopback_test`** — spawns `coop_tone.exe` (a standalone configurable WASAPI
  sine-wave source under [`tools/audio_tone`](tools/audio_tone)) at several source
  formats (device default, 44100/48000/96000 Hz) and verifies the shipping
  process-loopback capture (the fallback backend) receives non-silent audio by PID for
  each — confirming loopback is format-agnostic (it captures post-mix at the device
  endpoint format). Skips cleanly if the machine has no audio endpoint.
- **`mock_game_test`** — comprehensive capture/audio/hook stress test against
  **`coop_mock_game`** (an animated, frame-numbered A/V test game under
  [`tools/mock_game`](tools/mock_game) with selectable **DX9 / DX9Ex / DX10 / DX11 / DX12 /
  OpenGL / Vulkan** backends and a configurable WASAPI tone). It launches the game, injects
  `coop_hook.dll`, opens the hook's shared video texture, and **decodes the frame number out of
  the captured pixels** to assert the mirror sees a *monotonic, advancing* sequence for each
  backend (the bar for no dropped / stale / out-of-order frames — what the DX12 rotating-backbuffer
  bug broke). **Vulkan** is special: since its present pointer is cached at init, the test launches
  the mock **suspended**, injects, then resumes (the mock loads Vulkan and waits so the hook arms
  first) to decode frames; it also exercises the **implicit capture layer** (registered via
  `VK_LAYER_PATH` / `VK_INSTANCE_LAYERS`, decoding frames through the loader-inserted layer) and
  late-injects a Vulkan game to assert the **too-late** flag trips (which drives the host's
  relaunch banner). It launches the game at several **audio formats**
  (44100/48000/96000, PCM + float) and asserts the hook measures each one's rate through the full
  inject path, then injects with audio + video, checks both stream, cycles the audio subsystem
  off/on (hook/unhook stress), and confirms the game never crashes and capture resumes. This suite
  drove out five real audio races (see Lessons learned). Skips cleanly without a D3D11 / Vulkan
  device.

### Debugging the hooks against a real game

[`tools/audio_probe`](tools/audio_probe) (`coop_audio_probe.exe <pid> [seconds]`)
brings up the audio render-hook without Steam / RPT / the host UI: it creates the
IPC block + audio ring the hook expects, injects `coop_hook.dll` into the target
game, then drains the ring and prints per-stream format, captured-frame counts,
peak amplitude (proves the audio is real, not silence), and overruns. It enables
the hook's file trace (`%TEMP%\coop_hook.log`) for the run.

[`tools/audio_validate`](tools/audio_validate) (`coop_audio_validate.exe`) quantifies
audio-capture **fidelity** — it turns "the audio sounds slightly off" into numbers. With no
args it plays a known sine (`coop_tone` at 44100 Hz on a 48000 Hz endpoint, the Godot/Brotato
case), injects the hook exactly as the host does (late attach), captures the ring, and runs the
analyzer ([`common/include/coop/tone_analysis.hpp`](common/include/coop/tone_analysis.hpp)):
**pitch error in cents** (detects a mis-measured rate), **SNR/THD**, and **click / dropout**
counts. It also dumps a `.wav` so the capture can be *listened* to. Modes: `--render` drives the
real `AudioMirror` and measures its *rendered* output (surfaces the under-run / re-prime gaps the
capture side can't show — see Lessons learned); `--baseline` / `--selfcheck` establish the
measurement floor; `--listen <pid>` passively records a live process's output (point it at a
running `coop_host` to hear/quantify exactly what a guest gets on a real game); `--wav <file>`
analyzes any recording.

[`tools/input_probe`](tools/input_probe)
(`coop_input_probe.exe <pid> [seconds] [disable_mask]`) does the same for input: it
injects, reports one connected pad, and toggles a button each second so the game's
input layer sees a real state change. `disable_mask` (hex bits `0x1`=input
`0x2`=focus `0x4`=audio `0x8`=video) skips installing a subsystem, so you can
**bisect which injected subsystem affects a game** — this is how the 32-bit
Present-hook crash was isolated.

Both auto-detect a 32-bit (WOW64) target and inject via `coop_inject_x86.exe` +
`coop_hook_x86.dll`, exactly like the host. The probes build into
`bin/<config>/tools/` (the deployable `bin/<config>/` root holds only shipping
artifacts; tests build into `bin/<config>/tests/`) and resolve `coop_hook.dll` from
the root one level up, so run them from there. **Kill the game between runs** — the
loaded DLL locks `coop_hook.dll` against the next rebuild.

A debug-only **test harness** drives the host overlay's own code paths (inject / enable
audio / re-measure / override / screenshot / read state) without simulating mouse input,
for scripted UI validation. Build it with `-DCOOP_TEST_HARNESS=ON` (off by default, so
the shipped host never contains it); the host then reads one command line from
`%TEMP%\coop_test_cmd.txt` and replies in `%TEMP%\coop_test_resp.txt`. See
[`host/src/test_harness.hpp`](host/src/test_harness.hpp).

## Running the tool (manual, end-to-end)

This needs Steam, a donor game that supports Remote Play Together, and a second
person/account to receive the stream.

1. **Launch the host under a donor appid.** Find the donor's appid (the number in
   its store URL); the donor only needs RPT support and is never actually played:

   ```text
   "C:\Program Files (x86)\Steam\steam.exe" -applaunch <donorAppId> "D:\dev\CoopAllTheThings\bin\Debug\coop_host.exe"
   ```

   The borderless window appears and Steam marks the donor "running". If the donor
   ignores the trailing path, set the host as the donor's **Launch Options**
   (`"D:\...\coop_host.exe" %command%`) or use a launcher like RemotePlayDetached.

2. **Start the real game** windowed or borderless (not exclusive fullscreen — see
   Lessons learned). In the host's **Injection** panel, filter for the game's
   `.exe`, select it, and click **Inject & Connect**. Watch **Hook status** for
   **Attached**, a non-zero **XInput polled: N/s**, and **Focus spoof: active**.

3. **Mirror video:** in the **Video mirror** panel, tick **Mirror game window** —
   the host window now shows a live, letterboxed copy of the game. **Source**
   picks how the frames are grabbed: **WGC** (default, Windows Graphics Capture —
   works for any window) or **Hooked (Present)** (the injected hook's shared
   texture — lower latency and no capture border, for DXGI / D3D11 and OpenGL
   games; selecting it installs the video subsystem in the game).

4. **Mirror audio:** in the **Audio mirror** panel, tick **Mirror game audio**.
   With the hook injected, **Source** shows **Hooked (no echo)** and the game's
   local playback goes silent while guests still hear it. If it shows **Loopback
   (echo)** the hook's render path wasn't caught and you'll hear the game twice
   locally (guests still hear it once). The **Render streams** table shows how
   many WASAPI streams the game emits.

5. **Start Remote Play Together** from Steam and invite a guest. Verify the guest
   sees the mirrored video, hears the audio, and that their controller drives the
   real game.

Useful checks while developing without RPT: tick **Forward synthetic test input**
in the Injection panel to make the game move on its own (proving forwarding is the
source), and click away from the game to confirm focus spoofing keeps it running.

> Injection access error → run the host as administrator. A 32-bit (WOW64) target
> is injected automatically via `coop_inject_x86.exe` + `coop_hook_x86.dll`; if
> those aren't next to the host, rebuild (the x86 sub-build stages them there).

## Lessons learned

Non-obvious things that cost time and constrain the design:

- **RPT only streams the *focused* window.** The game can't hold focus itself, so
  the hook spoofs it (`GetForegroundWindow` / `GetActiveWindow` / `GetFocus` +
  swallowing deactivation messages) to keep the game polling and rendering while
  the host owns real OS focus.
- **Run target games windowed or borderless, never exclusive fullscreen** —
  exclusive fullscreen minimizes on focus loss (defeating the spoof) and can't be
  window-captured. While unfocused the game gets no OS keyboard/mouse, only the
  forwarded pad.
- **WGC captures occluded windows but not minimized ones.**
- **Process-loopback capture doesn't mute the source.** Capturing a process's
  render doesn't stop it reaching the speakers, so the no-echo path instead injects
  a WASAPI render-hook that copies each buffer then releases it with
  `AUDCLNT_BUFFERFLAGS_SILENT`; loopback stays as the (echoing) fallback.
- **`ActivateAudioInterfaceAsync` needs an *agile* completion handler.** If the
  handler doesn't answer `QueryInterface` for `IAgileObject`, the call is rejected
  **synchronously** with `E_ILLEGAL_METHOD_CALL` (`0x8000000E`) — regardless of
  apartment, device path, or activation params. (WRL/wil samples make the handler
  agile for you.) Process loopback also needs the Win10 20H1 headers
  (`NTDDI_VERSION ≥ 0x0A00000B`).
- **COM methods have no exports, so hooks walk vtables by frozen-ABI index — count
  exactly.** All instances of a coclass share one vtable, so hooking one object's
  slot catches every instance; but `IAudioClient::GetService` is **14**, not 13
  (`SetEventHandle` sits at 13 between `Reset` and `GetService`). Count every
  inherited `IUnknown`/base method when adding a hook.
- **D3D12 capture copies the *rotating* back buffer, not `GetBuffer(0)`.** D3D11
  flip-model keeps `GetBuffer(0)` pointing at the live back buffer, but D3D12 rotates
  buffers explicitly — the game renders into the buffer at
  `IDXGISwapChain3::GetCurrentBackBufferIndex()`, which advances each `Present`.
  Grabbing buffer 0 copies a stale buffer on N-1 of every N frames, so the mirror
  silently runs at refresh/N — yet the Present counter, published FPS, generation, and
  latency all read full rate (they count Presents, not unique content), so the metrics
  look perfect while the eye sees missing frames. Query the current index right before
  the trampoline `Present` (that's the just-rendered buffer) and copy that one.
- **Keep the D3D12 capture copy off the game's present queue, but ordered after its
  frame.** The D3D11 path copies on the game's immediate context, so it's naturally
  ordered after the frame and on the game's own timeline. For D3D12 the D3D11On12
  bridge needs a queue: running the copy on the *game's* present queue orders it
  correctly but stalls the game's own presents (GPU back-pressure, plus the shared
  keyed-mutex `AcquireSync` is a **CPU-blocking** call on the render thread). Running
  it on an independent queue avoids the stall but races the game's render → stale
  frames. The fix is both: run the copy on **our own** queue, and order it with a
  **fence** the game's present queue signals after its frame (a near-free op) and our
  queue waits on. The present queue is recovered for late injection by hooking
  `ID3D12CommandQueue::ExecuteCommandLists` (the per-frame method, not creation). Make
  the producer-side `AcquireSync` non-blocking (`timeout 0`) so a busy mutex drops a
  *mirror* frame instead of stalling the game; the Video panel's "Frames lost" line
  surfaces both capture- and display-stage drops.
- **A D3D10 game's backbuffer lies about being D3D11.** A pure-D3D10 swapchain's backbuffer
  QIs to `ID3D11Texture2D` *successfully*, but that D3D11 view reads back **empty** — the
  rendered content only exists on the game's own D3D10 device. (And a D3D11 backbuffer QIs to
  `ID3D10Texture2D` too, so `GetBuffer` alone can't tell them apart.) The reliable signal is
  that a feature-level-10 device **rejects** `CreateTexture2D` with the NT-handle keyed-mutex
  share flags (`E_INVALIDARG`): so try the D3D11 fast path, and on that failure switch (sticky)
  to reading the backbuffer through the game's **D3D10** device into a staging texture and
  `UpdateSubresource`-ing it into the shared texture on a **hook-owned D3D11 device** (the game
  has no usable D3D11 device of its own). The staging `Map` blocks until the GPU copy completes,
  so there's no cross-device race.
- **D3D9 capture: one read-back path covers plain D3D9 *and* D3D9Ex.** D3D9 has no DXGI
  swapchain, so inline-hook `IDirect3DDevice9::Present` (vtable index **17**, found from a
  throwaway device — clean prologue, so inline is safe; `stdcall()` on x86). A D3D9 surface
  isn't D3D11-shareable, so read the backbuffer back with `GetRenderTargetData` into a
  `D3DPOOL_SYSTEMMEM` surface (`LockRect` blocks until the copy → no race), **swizzle BGRA→RGBA**
  (`X8R8G8B8`/`A8R8G8B8` store as little-endian `0xAARRGGBB` → bytes B,G,R,A) and upload into the
  shared texture on a hook-owned D3D11 device. Both plain D3D9 and D3D9Ex call this same Present,
  so **one path serves both** — the GPU shared-surface fast path the plan sketched for D3D9Ex
  wasn't worth it (D3D9 games are light enough that the read-back cost is fine, and it dodges the
  cross-device keyed-mutex-less sync of a legacy shared surface).
- **The OpenGL mock needs no GL loader.** Drawing the animated pattern with scissored clears
  (`glClear` + `glScissor`) only touches **GL 1.1**, which `opengl32` exports directly — so the
  planned **glad** submodule wasn't needed (a legacy `wglCreateContext` + `<GL/gl.h>` suffices).
  GL's framebuffer is **bottom-left** origin and the capture flips it top-down, so the mock draws
  the frame-counter block at the GL *top* (`y = h - block`) to land at the captured image's
  top-left. The GL `SwapBuffers` hook now also bumps the shared present counter (it's the GL
  present), so the Video panel's present rate works for GL games too.
- **Vulkan can't be late-hooked, and capturing its present needs semaphore surgery.** Vulkan
  games cache `vkQueuePresentKHR` at init (often via volk, which bypasses the loader trampoline),
  so late injection misses it — the hook must be in **before `vkCreateInstance`**. Catch the
  resolution chain instead: inline-hook the `vulkan-1.dll` `vkGetInstanceProcAddr` export and hand
  back wrappers for `vkCreateInstance` / `vkCreateDevice` / `vkCreateSwapchainKHR` /
  `vkQueuePresentKHR` (so a volk app resolves *ours*), tracking the device / queue / images /
  format; read the presented image back with `vkCmdCopyImageToBuffer` and upload it to the shared
  texture on a hook-owned D3D11 device. The trap: the read-back submit must **wait on the present's
  wait semaphores** (consume them) and **signal a fresh semaphore the real present then waits on**
  — both your copy and the present can't wait the same binary semaphore. "Injected too late" is
  detected as *vulkan-1.dll loaded + hook in for >4 s + we never saw `vkCreateDevice`* (the app set
  everything up before us) → red relaunch banner, WGC meanwhile. Testing needs an **early-load**
  path (suspended launch + inject + resume; the mock loads Vulkan and waits), since the normal
  late-inject flow can't catch a Vulkan present.
- **The reliable Vulkan path is a real implicit layer — and the loader tags its link structs with
  small `sType`s.** Games that init Vulkan instantly resolve their present pointer before any
  inject can land, so the only robust early presence is an implicit layer (`coop_vk_layer`) the
  loader inserts at `vkCreateInstance`. It must do **chain dispatch** (pull
  `pfnNextGetInstanceProcAddr` / `pfnNextGetDeviceProcAddr` out of the `VkLayer*CreateInfo` link in
  `pNext`, advance the link, call down) rather than inline-hook. The trap that cost time: those
  link structs use **loader-internal `sType` values 47 (instance) / 48 (device)** — *not* the
  `1000000000`-range I assumed (and `vk_layer.h` isn't in Vulkan-Headers, so the structs are
  hand-declared) — matching the wrong value silently failed the device-chain walk
  (`vkCreateDevice: Failed to create device chain`). An implicit layer loads into *every* Vulkan
  app, so it self-scopes: capture only when the process image matches the host-written target file,
  else pure pass-through. Register it per-user (HKCU `…\Vulkan\ImplicitLayers`, no admin) and
  unregister on untick / host exit.
- **A render client that predates our injection has no knowable format — measure it.**
  We inject into already-running games, so we usually never see the game's
  `IAudioClient::Initialize`; the render-hook then assumes the device mix format for that
  stream. That's wrong for games that render at a non-device rate via WASAPI
  `AUTOCONVERTPCM` (e.g. Godot / Brotato render 44100 Hz while the endpoint mixes at
  48000), so the captured audio plays back **pitch-shifted up**. Fix: treat such a format
  as a *guess* and measure the stream's true sample rate from its render cadence
  (frames/sec over a short active window, snapped to the nearest standard rate) before
  publishing it, deferring capture until verified. Discard the first measurement window:
  the moment we attach, the stream's already-queued buffers arrive in a burst that
  over-counts (the `audio_hook_test` matrix caught this), so measure the next,
  steady-state window. **Only the rate is recoverable, though** — channels/bit-depth can't
  be measured (`AUTOCONVERTPCM` hands `GetBuffer` a *fixed* staging buffer, so there's no
  buffer stride; confirmed empirically) and WASAPI has no API for a pre-existing client's
  format, so they stay the device-mix guess. That's right for the common case (engines
  render stereo float = the endpoint), and a `VirtualQuery` clamp on the capture copy
  keeps a too-large guessed block from ever over-reading the source buffer. Streams we *do*
  watch get created carry their exact `Initialize` format, and the loopback fallback
  captures post-mix at the device format (always correct). The Audio panel shows each
  stream's provenance (known / measuring / measured rate (ch/bits assumed)) so what the
  mirror is using is always visible — see Limitations.
- **A short measurement window *rejects* a bad reading, it can't *average* it away.**
  Measuring the rate over ~200 ms made one extra ~10 ms WASAPI buffer at a window edge a
  ~5% error, which lands *between* standard rates (they're >8% apart) — so 44100 read as
  ~46205 and got published verbatim. The robust fix isn't just a longer window: it's to
  **refuse any window that doesn't snap to a standard rate** and require **consensus**
  across windows, since a quantization/burst error big enough to miss the right rate
  lands in no-man's-land rather than on a wrong neighbour. Only commit a non-standard
  estimate as explicitly *low-confidence* (shown red). The operator can also re-measure
  or override the format via a per-stream `AudioRingHeader` op channel; the host rebuilds
  its render client when `format_generation` bumps, so it takes effect live.
- **Re-priming the render feed on a *partial* fill manufactures the gap it's avoiding.** The
  mirror re-renders the captured ring to the output device. The original feed loop re-primed
  (withheld the feed until ~30 ms had rebuffered) whenever it couldn't completely fill the free
  buffer that tick (`to_write < avail`). But a partial fill is *normal* producer jitter — and
  withholding the feed drains the device, so a one-frame ring dip became a full ~30 ms drop-out.
  On a jittery game it fired constantly → choppy, "metallic" mirror audio, intermittent by
  buffer phase. Fix: feed whatever is available every tick; re-prime **only** on a genuine
  starvation (device buffer empty *and* ring empty). The policy is factored into a pure
  `RenderPacer` reused by the loop and unit-tested headlessly (`render_pacer_test`: on a jittery
  schedule the old policy withholds available data ~168× and drains the cushion to one period
  from silence; the new one never withholds). **Diagnosing it needed device-side capture, not a
  write-side tap** — the artifact is silence the device *plays* during an under-run, not bytes
  the loop writes, so a tap on the write would look clean. `coop_audio_validate` quantifies it
  by loopback-capturing the *rendered* output (pitch error in cents, SNR, click/dropout counts)
  and contrasts the capture ring (pristine), the measurement floor, and the render path.
- **Mute the game with the SILENT flag, not a `memset` — they are not the same thing.** The
  hooked path's whole point is "no echo": capture the game's frames into the ring AND silence
  its local playback, so the only audio is the host's re-render. The mute was implemented by
  *zeroing the buffer* and releasing it with `AUDCLNT_BUFFERFLAGS_SILENT`. But zeroing
  `num_frames * block` is only safe when `block` is the real frame size — for a **guessed**
  format (late attach, the Brotato case) the guessed block can exceed the real buffer, so the
  zero would over-write adjacent memory. The original code's conservative answer was to skip the
  whole mute for guessed streams — which left the game **audible**: it played locally *and* the
  mirror re-rendered the same audio a few ms later = a metallic, slightly-out-of-sync double
  (exactly the reported symptom). The fix: `AUDCLNT_BUFFERFLAGS_SILENT` already makes WASAPI
  *ignore the buffer contents* and play silence — it mutes with **no write at all**, so it's safe
  for any format. Decouple the two: always mute via the flag; keep the `memset` only for an exact
  format (belt-and-suspenders). `audio_hook_test` now asserts the mute engages for both the
  exact and the guessed path (the guessed assertion fails on the old code — write the failing
  test first), and the byte-incompatible cases prove the flag-mute never over-writes.
- **Panels must *fit* their assigned size at max info -- measure it, don't eyeball it.** The
  overlay opens panels at fixed sizes that scale with the monitor, so with Debug details on a
  dense panel can overflow and scroll content out of view. `ui_fit_test` drives the real
  Controllers + Audio panels headlessly (null-backend ImGui: build the atlas with
  `GetTexDataAsRGBA32`, set a non-null `TexID`, `Render()` needs no GPU) at reference
  resolutions and asserts each window's `ScrollMax` is 0 -- read it on the *2nd+* frame, since
  ImGui computes `ScrollMax` in `Begin()` from the *previous* frame's content size. Two fixes
  fell out: the center column was too narrow (horizontal overflow -> widen it, merge the
  Controllers poll/round-trip tables), and a *static* height split can't serve both modes, so
  it's now **Debug-details-aware** (debug on -> Controllers/Audio get the height for their
  tables; off -> Video's perf graphs are the tall content, since Video's height is
  mirroring-driven, not debug-driven). The host's `uisize`/`uifit` harness commands check the
  same thing against the live overlay.
- **Drive the ImGui overlay for tests through an IPC harness, not synthetic input.**
  `PostMessage`-d mouse clicks don't reliably reach ImGui widgets, and key/coordinate
  simulation is brittle. A tiny debug-only command channel (`-DCOOP_TEST_HARNESS`, file-
  based) that calls the *same* code the buttons do — and replies with state — makes UI
  validation deterministic and scriptable, and is compiled out of the shipped product.
- **A capture-style stress test against a frame-numbered mock game is worth a lot.** An
  animated game that encodes its frame number in the pixels lets a test assert the mirror
  shows a *monotonic, advancing* sequence (the bar for "no dropped / stale / out-of-order
  frames"), and toggling subsystems while it renders flushes out concurrency bugs. This
  one caught **five** real audio races: a `memset` over-*write* past a guessed stream's
  real buffer (zeroing 8ch into a 2ch buffer corrupts adjacent audio memory → crash; the
  fix: capture but don't silence a guessed stream), a null call through a vtable hook's
  `original` after unhook (keep it valid), a non-atomic `g_ipc` TOCTOU, a stale
  GetBuffer/ReleaseBuffer pairing across a hook toggle (epoch-stamp it), and COM-object
  churn from re-creating the probe each toggle (build it once, keep it, only swap vtable
  slots). **Silently silencing/zeroing a buffer whose true size you only guessed is an
  over-write, not just an over-read** — clamp the read, but don't write what you can't size.
- **Capturing at `Present` decouples the mirror from DWM composition.** The hook copies
  the backbuffer inside the game's `Present`, which the game issues at its true render
  rate regardless of how DWM composites that *window*. So an unfocused game window can
  judder (DWM under-composites background windows; only the focused window gets VRR /
  independent flip) while the mirror — which receives every `Present` — stays smooth.
  This is why the game window not being focused doesn't matter: it isn't the surface
  anyone sees. The same focus rule explains why an unfocused *tool* window can render
  below the game's rate (it loses VRR), so in use the mirror is the focused window.
- **SafetyHook on x86 has two traps that froze 32-bit Slaps and Beans.** (1)
  `InlineHook::call()` invokes the trampoline as `__cdecl`, but most targets are
  `__stdcall` (COM methods like `IDXGISwapChain::Present`, the WASAPI interfaces,
  `WINAPI` `SwapBuffers`); on 32-bit that double-cleans the stack → ESP imbalance →
  crash (Debug: **Run-Time Check Failure #0**). Use **`stdcall()`** (a no-op on
  x64). (2) Don't *inline-hook* COM methods on x86 at all: MMDevApi/AudioSes
  prologues do `push ebp; mov ebp,esp; and esp,-8` and read args **EBP-relative**,
  which SafetyHook's trampoline relocation breaks (the original then runs with
  garbage args and faults). Hook COM methods by **swapping the vtable entry**
  instead (`VirtualProtect` the slot, overwrite the pointer, call the saved
  original) — no code patching, pristine stack regardless of prologue. Inline
  hooking stays fine for `Present`/`SwapBuffers` (clean prologues). Guarded by the
  x86 hook tests.
- **Steam Input init suppresses XInput.** Initializing Steam Input turns on Steam's
  in-process XInput interception, which hides controllers from `XInputGetState`
  unless they're bound to the running appid's action set — defaulting to it
  silently broke forwarding. XInput is primary; Steam Input is opt-in.