feat(windows): mpv draws Windows video into the app window

DR-237's video half. mpv now renders Windows video the way
tauri-plugin-libmpv does on Windows: MpvBackend is handed the main
window's HWND and sets it as `wid` before mpv initialises, so mpv draws
as a child of the app window beneath the transparent WebView2, with
vo=gpu-next,gpu. osc, default bindings, VO keyboard and cursor handling
are off, so the Svelte controls drawn over the picture are the only ones.

- video_output() decides per platform (UT-274): Window(hwnd) on Windows,
  RenderApi on Linux, Off without native video. Windows with no handle
  draws nothing rather than letting mpv open a top-level window.
- native_video::enabled() is true on Windows as well, so the frontend
  takes the native path there: NativePlayerAdapter, and the page clears
  its background while video is on screen.
- enableNativeVideoCompositing() no longer logs a missing Android bridge
  as an error on the desktop, where there is no bridge to have.

Verified: every option, wid included, is accepted by the real libmpv on
Linux and by the shipped Windows DLL under wine; the Windows unit suite
passes under wine (949). Not yet seen on real Windows hardware.

docs/build/build-windows.md

@@ -7,9 +7,15 @@ job / SMTC lockscreen), but it runs and plays media.
 
 ## How playback works on Windows
 
-- **Video** — renders through the webview HTML5 `<video>` element (hls.js); on
+- **Video** — **mpv**, drawing into the app's own window: `MpvBackend` is
-  Windows that is WebView2 (Chromium/Edge), which plays HLS + h264 fine. mpv
+  handed the main window's HWND as `wid` before mpv initialises and renders with
-  takes this over in DR-237's video phase; Linux already made that move.
+  `vo=gpu-next,gpu` as a child of it, beneath the transparent WebView2 whose page
+  clears its background while a video is on screen (`data-native-video`). mpv's
+  own controller, key bindings and cursor handling are off — the Svelte controls
+  drawn over it are the only ones. This is the arrangement tauri-plugin-libmpv
+  ships on Windows (same zhongfly LGPL DLL). **Not yet seen on real Windows
+  hardware** — if the picture ends up *over* the controls, the z-order of mpv's
+  child window is the first thing to check.
 - **Audio** — **libmpv**, the same `MpvBackend` Linux uses, with `ao=wasapi`.
   Volume, EQ, normalization and gapless all go through mpv's filter graph as on
   Linux. `libmpv-2.dll` ships beside `jellytau.exe` in the installer.
@@ -87,4 +93,4 @@ Outputs:
 
 1. SMTC (lockscreen / media keys) — not wired on Windows.
 2. Code signing — the installer is unsigned.
-3. Video through mpv (DR-237) — needs a WebView2-side surface.
+3. First run of mpv video on real Windows hardware (DR-237).

docs/requirements.md

@@ -869,6 +869,7 @@ Internal architecture, components, and application logic.
 | UT-271 | Native video is on for Linux whatever `JELLYTAU_NATIVE_VIDEO` says, including unset and explicit "off" values, and off where mpv is not the video renderer | DR-235 | Done |
 | UT-272 | No platform reports a webview video fallback, Linux reports native video, and the player status on Linux never sends video to the `<video>` element | DR-235 | Done |
 | UT-273 | `player_play_item`, `get_player_status` and `player_get_capabilities` answer "does a native renderer draw video here" from one function, so the backend is loaded with video exactly where the frontend is told not to use a `<video>` element — on Windows, where mpv now plays audio, the film's soundtrack is not decoded twice | DR-237 | Done |
+| UT-274 | mpv's video output is decided per platform: Windows renders into the app window's HWND (`wid`, set before initialisation) with `vo=gpu-next,gpu` and mpv's own controller, bindings and cursor handling off; Windows with no handle draws nothing rather than opening a window of its own; Linux uses the render API; no native video means `video=no`. Every option is accepted by the real libmpv, on Linux and by the shipped Windows DLL | DR-237 | Done |
 ### Integration Tests
 
 | Test ID | Test Description | Traces To | Status |

src-tauri/src/conformance_runner.rs

@@ -150,6 +150,7 @@ pub fn run_engine(url: &str, engine: Engine) -> u32 {
                     None,
                     std::sync::Arc::new(tokio::sync::Mutex::new(None)),
                     std::sync::Arc::new(crate::playback_reporting::throttle::EventThrottler::new()),
+                    None,
                 )
                 .expect("could not create the legacy backend"),
                 crate::player::media_player::Capabilities::mpv(),

src-tauri/src/lib.rs

@@ -699,7 +699,25 @@ fn create_player_backend(
     #[cfg(any(target_os = "linux", target_os = "windows"))]
     {
         info!("Initializing MPV backend");
-        match MpvBackend::new(Some(_event_emitter), playback_reporter, position_throttler) {
+        // Windows: mpv draws video into the main window itself (DR-237), so it
+        // needs the HWND before it initialises. Linux draws through the render
+        // API into a GTK surface attached later, and needs no handle.
+        #[cfg(target_os = "windows")]
+        let video_window = {
+            use tauri::Manager;
+            app_handle
+                .get_webview_window("main")
+                .and_then(|w| w.hwnd().ok())
+                .map(|hwnd| hwnd.0 as i64)
+        };
+        #[cfg(not(target_os = "windows"))]
+        let video_window = None;
+        match MpvBackend::new(
+            Some(_event_emitter),
+            playback_reporter,
+            position_throttler,
+            video_window,
+        ) {
             Ok(backend) => {
                 info!("Successfully initialized MPV backend");
                 Box::new(backend)

src-tauri/src/player/mpv_backend.rs

@@ -149,12 +149,73 @@ pub fn registered_handle() -> *mut libmpv_sys::mpv_handle {
         .unwrap_or(std::ptr::null_mut())
 }
 
+/// How mpv shows video on this platform.
+///
+/// TRACES: UR-080 | DR-231, DR-237
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub(crate) enum VideoOutput {
+    /// No picture: audio-only playback, or nowhere to draw.
+    Off,
+    /// Linux: frames through the render API into the GTK surface beneath the
+    /// webview (`video_surface`).
+    RenderApi,
+    /// Windows: mpv renders as a child of the app's own window (`wid`, set
+    /// before initialisation), beneath the transparent WebView2 — the
+    /// arrangement tauri-plugin-libmpv ships on Windows.
+    Window(i64),
+}
+
+impl VideoOutput {
+    /// Runtime options for this output. `wid` is not among them: it only takes
+    /// effect before initialisation, so the constructor sets it separately.
+    pub(crate) fn options(&self) -> Vec<(&'static str, String)> {
+        match self {
+            VideoOutput::Off => vec![("video", "no".to_string())],
+            VideoOutput::RenderApi => vec![("vo", "libmpv".to_string())],
+            VideoOutput::Window(_) => [
+                // libplacebo's renderer, with the classic one as fallback for a
+                // build or GPU that lacks it.
+                ("vo", "gpu-next,gpu"),
+                // mpv is a surface here, not a player: the app's controls are
+                // drawn over it, so its own controller and bindings must not
+                // answer clicks, keys or the cursor.
+                ("osc", "no"),
+                ("input-default-bindings", "no"),
+                ("input-vo-keyboard", "no"),
+                ("input-cursor", "no"),
+                ("cursor-autohide", "no"),
+            ]
+            .into_iter()
+            .map(|(k, v)| (k, v.to_string()))
+            .collect(),
+        }
+    }
+}
+
+/// Decide the video output from whether native video is on, the platform, and
+/// the app window's handle (Windows only).
+///
+/// TRACES: UR-080 | DR-231, DR-237 | UT-274
+pub(crate) fn video_output(native: bool, is_windows: bool, window: Option<i64>) -> VideoOutput {
+    match (native, is_windows, window) {
+        (false, _, _) => VideoOutput::Off,
+        (true, true, Some(wid)) => VideoOutput::Window(wid),
+        // No handle: mpv would open a top-level window of its own.
+        (true, true, None) => VideoOutput::Off,
+        (true, false, _) => VideoOutput::RenderApi,
+    }
+}
+
 impl MpvBackend {
     /// Create a new MPV backend
+    ///
+    /// `video_window` is the app window's native handle (an HWND), which mpv
+    /// draws video into on Windows; `None` elsewhere.
     pub fn new(
         event_emitter: Option<Arc<dyn PlayerEventEmitter>>,
         playback_reporter: Arc<TokioMutex<Option<PlaybackReporter>>>,
         position_throttler: Arc<EventThrottler>,
+        video_window: Option<i64>,
     ) -> Result<Self, PlayerError> {
         info!("[MpvBackend] Initializing MPV backend...");
 
@@ -166,7 +227,22 @@ impl MpvBackend {
             libc::setlocale(libc::LC_NUMERIC, c_locale.as_ptr());
         }
 
-        let mpv = Mpv::new().map_err(|e| PlayerError {
+        let output = video_output(
+            super::native_video::enabled(),
+            cfg!(target_os = "windows"),
+            video_window,
+        );
+        if super::native_video::enabled() && output == VideoOutput::Off {
+            error!("[MpvBackend] no window handle to draw video into; video will have no picture");
+        }
+        // `wid` only takes effect before initialisation. TRACES: UR-080 | DR-237
+        let mpv = Mpv::with_initializer(|init| {
+            if let VideoOutput::Window(wid) = output {
+                init.set_property("wid", wid)?;
+            }
+            Ok(())
+        })
+        .map_err(|e| PlayerError {
             message: format!("Failed to initialize MPV: {:?}", e),
         })?;
         // TRACES: UR-012 | DR-299
@@ -203,24 +279,21 @@ impl MpvBackend {
         //
         // Linux video went through the webview until DR-235, and decoding it
         // here too would have burned a core for a picture nobody saw — hence
-        // `video: no`. With native video, mpv needs both the decoder *and*
+        // `video: no`. With native video, mpv needs the decoder *and* an output
-        // `vo=libmpv` — the render API only works through that output, and the
+        // that draws where the app wants it: the render API on Linux (the default
-        // default would try to open a window of its own.
+        // would open a window of its own), the app's window on Windows.
         //
         // Set at construction because mpv resolves the video output when it
         // initialises; flipping it later does not re-open one.
         //
-        // TRACES: UR-080 | DR-231, DR-235
+        // TRACES: UR-080 | DR-231, DR-235, DR-237
-        if super::native_video::enabled() {
+        for (name, value) in output.options() {
-            mpv.set_property("vo", "libmpv").map_err(|e| PlayerError {
+            mpv.set_property(name, value.as_str())
-                message: format!("Failed to select the libmpv video output: {:?}", e),
+                .map_err(|e| PlayerError {
-            })?;
+                    message: format!("Failed to set {name}={value}: {:?}", e),
-            info!("[MpvBackend] native video enabled (vo=libmpv)");
-        } else {
-            mpv.set_property("video", "no").map_err(|e| PlayerError {
-                message: format!("Failed to configure MPV video: {:?}", e),
                 })?;
         }
+        info!("[MpvBackend] video output: {:?}", output);
 
         // Set volume to 100% (we'll control via MPV's volume property)
         mpv.set_property("volume", 100i64)
@@ -1023,3 +1096,97 @@ mod af_filter_tests {
         assert!(eq_pos < norm_pos, "normalizer runs after EQ: {af}");
     }
 }
+
+#[cfg(test)]
+mod video_output_tests {
+    use super::{video_output, VideoOutput};
+
+    /// Windows draws into the app's own window: mpv is handed its HWND before
+    /// initialising and renders as a child of it, beneath the transparent
+    /// WebView2.
+    ///
+    /// TRACES: UR-080 | DR-237 | UT-274
+    #[test]
+    fn windows_native_video_renders_into_the_app_window() {
+        assert_eq!(
+            video_output(true, true, Some(0x1234)),
+            VideoOutput::Window(0x1234)
+        );
+    }
+
+    /// Without a handle mpv would open a top-level window of its own, a second
+    /// window floating beside the app. No picture is the honest failure.
+    ///
+    /// TRACES: UR-080 | DR-237 | UT-274
+    #[test]
+    fn windows_without_a_window_handle_draws_nothing() {
+        assert_eq!(video_output(true, true, None), VideoOutput::Off);
+    }
+
+    /// Linux keeps the render API the GTK surface draws from.
+    ///
+    /// TRACES: UR-080 | DR-231 | UT-274
+    #[test]
+    fn linux_native_video_uses_the_render_api() {
+        assert_eq!(video_output(true, false, None), VideoOutput::RenderApi);
+    }
+
+    /// TRACES: UR-080 | DR-231 | UT-274
+    #[test]
+    fn no_native_video_decodes_no_picture() {
+        assert_eq!(video_output(false, true, Some(1)), VideoOutput::Off);
+        assert_eq!(video_output(false, false, None), VideoOutput::Off);
+    }
+
+    /// In the app's window mpv must not act as a player of its own: its
+    /// on-screen controller and key/mouse bindings would compete with the
+    /// Svelte controls drawn over it.
+    ///
+    /// TRACES: UR-080 | DR-237 | UT-274
+    #[test]
+    fn a_window_output_hands_all_input_to_the_app() {
+        let opts = VideoOutput::Window(7).options();
+        for (k, v) in [
+            ("vo", "gpu-next,gpu"),
+            ("osc", "no"),
+            ("input-default-bindings", "no"),
+            ("input-vo-keyboard", "no"),
+            ("input-cursor", "no"),
+            ("cursor-autohide", "no"),
+        ] {
+            assert!(
+                opts.iter().any(|(ok, ov)| *ok == k && ov == v),
+                "missing {k}={v} in {opts:?}"
+            );
+        }
+        assert_eq!(
+            VideoOutput::Off.options(),
+            vec![("video", "no".to_string())]
+        );
+    }
+
+    /// Every option the outputs set is one this libmpv accepts, `wid` included —
+    /// against the real library, so a misspelt or removed option fails here
+    /// rather than as a player that will not start on a user's machine. Runs on
+    /// the Windows DLL too (under wine in the cross-build).
+    ///
+    /// TRACES: UR-080 | DR-237 | UT-274
+    #[test]
+    fn libmpv_accepts_every_video_output_option() {
+        let mpv = libmpv::Mpv::with_initializer(|init| {
+            init.set_property("wid", 0i64)?;
+            Ok(())
+        })
+        .expect("libmpv must accept wid before initialisation");
+        for output in [
+            VideoOutput::Off,
+            VideoOutput::RenderApi,
+            VideoOutput::Window(0),
+        ] {
+            for (name, value) in output.options() {
+                mpv.set_property(name, value.as_str())
+                    .unwrap_or_else(|e| panic!("libmpv rejected {name}={value}: {e:?}"));
+            }
+        }
+    }
+}

src-tauri/src/player/native_video.rs

@@ -16,31 +16,30 @@
 
 /// Whether mpv should decode and draw video in this process.
 ///
-/// True wherever mpv is the desktop video renderer — Linux, since DR-235
+/// True wherever mpv is the desktop video renderer: Linux since DR-235 phase 1,
-/// phase 1. There is no opt-out: the webview `<video>` path is no longer a Linux
+/// Windows since DR-237. There is no opt-out and no webview fallback — the
-/// video renderer, so "off" would leave nothing drawing the picture. It was the
+/// webview `<video>` path is gone, so "off" would leave nothing drawing the
-/// `JELLYTAU_NATIVE_VIDEO` opt-in while the render path was being proven; the
+/// picture. It was the `JELLYTAU_NATIVE_VIDEO` opt-in while the render path was
-/// variable is now ignored. Windows joins in DR-237, and only then does the
+/// being proven; the variable is ignored.
-/// webview path go (phase 3).
 ///
 /// TRACES: UR-080 | DR-231, DR-235
 pub fn enabled() -> bool {
     // On Android ExoPlayer draws video and `use_html5_element` is false for
     // entirely separate reasons.
-    cfg!(all(target_os = "linux", not(target_os = "android")))
+    cfg!(any(target_os = "linux", target_os = "windows"))
 }
 
 #[cfg(test)]
 mod tests {
     use super::*;
 
-    /// mpv draws video on Linux with nothing to opt into, and the retired
+    /// mpv draws video on Linux and Windows with nothing to opt into, and the retired
-    /// variable cannot opt back out: with the webview path gone from Linux, "off"
+    /// variable cannot opt back out: with the webview path gone, "off"
     /// would configure mpv for audio only with nothing else to draw the picture.
     ///
-    /// TRACES: UR-080 | DR-235 | UT-271
+    /// TRACES: UR-080 | DR-235, DR-237 | UT-271
     #[test]
-    fn linux_always_renders_video_natively() {
+    fn desktop_always_renders_video_natively() {
         let restore = std::env::var("JELLYTAU_NATIVE_VIDEO").ok();
 
         for value in [None, Some("0"), Some("false"), Some("1")] {
@@ -50,7 +49,7 @@ mod tests {
             }
             assert_eq!(
                 enabled(),
-                cfg!(all(target_os = "linux", not(target_os = "android"))),
+                cfg!(any(target_os = "linux", target_os = "windows")),
                 "JELLYTAU_NATIVE_VIDEO={value:?} must not decide the renderer"
             );
         }

src/lib/utils/videoSurface.ts

@@ -24,9 +24,18 @@
 
 import { nativeVideoActive } from "$lib/stores/nativeVideo";
 import { createLogger } from "$lib/utils/logger";
+import { platform } from "@tauri-apps/plugin-os";
 
 const log = createLogger("videoSurface");
 
+function isAndroid(): boolean {
+  try {
+    return platform() === "android";
+  } catch {
+    return false;
+  }
+}
+
 interface AndroidVideoSurfaceBridge {
   setTransparent(transparent: boolean): void;
   isSupported(): boolean;
@@ -69,6 +78,14 @@ export function enableNativeVideoCompositing(): void {
   // would see through the app to the home screen.
   nativeVideoActive.set(true);
   const androidVideoSurface = bridge();
+  // Only Android has a webview widget to make transparent. On the desktop the
+  // window is created transparent and mpv draws beneath it, so the page layer
+  // above is all there is — an absent bridge there is correct, not a fault.
+  // TRACES: UR-080 | DR-237
+  if (!androidVideoSurface && !isAndroid()) {
+    nativeVideoActive.set(true);
+    return;
+  }
   if (!androidVideoSurface) {
     // Say so loudly. Every bridge call in this file is optional-chained, so a
     // missing bridge is silent — and a silently-skipped setTransparent(true) is