fix(quality): scale after videoconvert at exact even WxH

Live medium-quality stream errored with "negotiation problem" on the
host and rendered a squashed, garbled picture in the viewer. Two causes,
both from inserting videoscale before videoconvert with PAR+range caps:

- videoscale was scaling pipewiresrc's raw output directly. The portal
  source's format/memory (e.g. DMABuf) isn't something software videoscale
  negotiates — the original pipeline always fed pipewiresrc through
  videoconvert first. Move videoscale *after* videoconvert so it operates
  on system-memory NV12/I420.
- `pixel-aspect-ratio=1/1` + a width range over-constrained negotiation
  and risked a non-square-PAR / distorted result. Instead compute an exact
  even WxH from the known source dimensions (Wayland: portal size; X11:
  root/window geometry), preserving aspect, and pin it fully in the caps.
  This is also downscale-only now — a source already at/below the target
  height is left native instead of upscaled. Unknown dims (rare X11
  geometry failure) fall back to the height-only + square-pixel + even
  width-range negotiation.

source_dims threaded through pipeline::spawn from both backends. Smoke
test updated to mirror the new ordering (1920x1080 -> 852x480, videoscale
after videoconvert) and still asserts an even sub-source width.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/host/pipeline.rs

@@ -66,17 +66,22 @@ impl Drop for CaptureHandle {
 
 /// Spawn the shared gst pipeline for a backend that supplies `source_args`
 /// (the video-source element + its properties, e.g. `["pipewiresrc", "fd=7",
-/// …]` or `["ximagesrc", "use-damage=false", …]`). `after_spawn` runs once,
-/// immediately after the gst child is launched — Wayland uses it to `close`
-/// the pipewire fd it leaked into the child; X11 passes a no-op.
+/// …]` or `["ximagesrc", "use-damage=false", …]`). `source_dims` is the source
+/// pixel size when the backend knows it (Wayland from the portal, X11 from
+/// root/window geometry); it lets a downscale preset compute an exact even
+/// target resolution and skip scaling when the source is already small enough.
+/// `after_spawn` runs once, immediately after the gst child is launched —
+/// Wayland uses it to `close` the pipewire fd it leaked into the child; X11
+/// passes a no-op.
 pub async fn spawn(
     opts: &HostOpts,
     quality: &EffectiveQuality,
+    source_dims: Option<(u32, u32)>,
     source_args: Vec<String>,
     after_spawn: impl FnOnce(),
 ) -> Result<CaptureHandle> {
     let (audio_routing, audio_device) = setup_audio(opts).await?;
-    let args = build_args(&source_args, &audio_device, opts, quality);
+    let args = build_args(&source_args, &audio_device, opts, quality, source_dims);
 
     let mut gst_cmd = Command::new("gst-launch-1.0");
     gst_cmd
@@ -148,6 +153,7 @@ fn build_args(
     audio_device: &str,
     opts: &HostOpts,
     quality: &EffectiveQuality,
+    source_dims: Option<(u32, u32)>,
 ) -> Vec<String> {
     let key_interval = (quality.framerate * 2).to_string();
     let bitrate = quality.bitrate.to_string();
@@ -187,9 +193,38 @@ fn build_args(
         "fd=1".into(),
     ];
 
+    // Downscale step for the quality presets. `None` = encode at native size
+    // (the "Source" preset, or a source already at/below the target height — we
+    // never upscale). When the source dimensions are known we pin an exact even
+    // WxH preserving the source aspect; H.264 4:2:0 needs even dims, so width is
+    // rounded to even and height is forced even (preset heights already are; a
+    // raw --max-height override is rounded down). When dims are unknown (a rare
+    // X11 geometry-read failure) we fall back to height-only + square pixels +
+    // an even-stepped width range and let videoscale negotiate.
+    let scale_caps: Option<String> = match quality.max_height {
+        None => None,
+        Some(max_h) => {
+            let h = (max_h & !1).max(2);
+            match source_dims {
+                Some((sw, sh)) if sh > h => {
+                    let w = ((sw as u64 * h as u64 + sh as u64 / 2) / sh as u64) as u32;
+                    let w = (w & !1).max(2);
+                    Some(format!("{raw_format},width={w},height={h}"))
+                }
+                Some(_) => None, // source already <= target height
+                None => Some(format!(
+                    "{raw_format},height={h},pixel-aspect-ratio=1/1,width=[2,8192,2]"
+                )),
+            }
+        }
+    };
+
     // video branch — videorate caps to the target fps so we don't ship at the
     // monitor's refresh rate (e.g. 180Hz) and pile up frames in the demuxer
-    // queue faster than realtime.
+    // queue faster than realtime. videoscale (when scaling) runs *after*
+    // videoconvert so it operates on system-memory NV12/I420: scaling
+    // pipewiresrc's raw output directly can hit a format/memory (e.g. DMABuf)
+    // that software videoscale won't negotiate.
     args.extend(source.iter().cloned());
     args.extend([
         "!".into(),
@@ -197,26 +232,6 @@ fn build_args(
         "!".into(),
         framerate_caps,
         "!".into(),
-    ]);
-    // Optional downscale (quality presets). Omitted entirely for the native
-    // "Source" preset. `pixel-aspect-ratio=1/1` forces a *proportional* scale:
-    // screen capture always has square pixels, and without pinning PAR videoscale
-    // keeps the full source width and just squashes PAR to preserve display
-    // aspect (e.g. 1920x480 @ PAR 4/9 — no bandwidth win at all). With square
-    // pixels fixed, width follows the source DAR. H.264 4:2:0 needs even
-    // dimensions, so we pin height to an even value (preset heights already are;
-    // a raw --max-height override is rounded down) and constrain width to a
-    // stepped even range — verified 1920x1080→852x480 and 1366x768→1280x720.
-    if let Some(h) = quality.max_height {
-        let h = (h & !1).max(2);
-        args.extend([
-            "videoscale".into(),
-            "!".into(),
-            format!("video/x-raw,height={h},pixel-aspect-ratio=1/1,width=[2,8192,2]"),
-            "!".into(),
-        ]);
-    }
-    args.extend([
         "queue".into(),
         "!".into(),
         "videoconvert".into(),
@@ -224,6 +239,9 @@ fn build_args(
         raw_format.into(),
         "!".into(),
     ]);
+    if let Some(caps) = scale_caps {
+        args.extend(["videoscale".into(), "!".into(), caps, "!".into()]);
+    }
     args.extend(encoder_args);
     args.extend([
         "!".into(),

src/host/wayland.rs

@@ -70,7 +70,7 @@ pub async fn start(opts: &HostOpts, quality: &EffectiveQuality) -> Result<Captur
         "do-timestamp=true".to_string(),
     ];
 
-    pipeline::spawn(opts, quality, source_args, move || {
+    pipeline::spawn(opts, quality, Some((w as u32, h as u32)), source_args, move || {
         // Parent no longer needs the pipewire fd — gst inherited its own copy.
         let _ = close(raw_fd);
     })

src/host/x11.rs

@@ -21,13 +21,21 @@ pub async fn start(opts: &HostOpts, quality: &EffectiveQuality) -> Result<Captur
         None
     };
 
-    // Geometry is informational (mirrors Wayland's portal-handshake log line);
-    // a failure here shouldn't abort capture — ximagesrc will surface a real
-    // error if the X connection is genuinely unusable.
-    match read_geometry(xid) {
-        Ok((w, h)) => tracing::info!(width = w, height = h, xid = ?xid, "X11 capture geometry"),
-        Err(e) => tracing::warn!("could not read X11 geometry (capture will still try): {e:#}"),
-    }
+    // Geometry mirrors Wayland's portal-handshake log line and feeds the
+    // downscale presets (so they can compute an exact target size). A failure
+    // here shouldn't abort capture — ximagesrc will surface a real error if the
+    // X connection is genuinely unusable, and the scaler falls back to a
+    // height-only negotiation when dims are unknown.
+    let source_dims = match read_geometry(xid) {
+        Ok((w, h)) => {
+            tracing::info!(width = w, height = h, xid = ?xid, "X11 capture geometry");
+            Some((w as u32, h as u32))
+        }
+        Err(e) => {
+            tracing::warn!("could not read X11 geometry (capture will still try): {e:#}");
+            None
+        }
+    };
 
     let mut source_args = vec![
         "ximagesrc".to_string(),
@@ -42,7 +50,7 @@ pub async fn start(opts: &HostOpts, quality: &EffectiveQuality) -> Result<Captur
     }
 
     // X11 has no leaked fd to clean up, so the post-spawn hook is a no-op.
-    pipeline::spawn(opts, quality, source_args, || {}).await
+    pipeline::spawn(opts, quality, source_dims, source_args, || {}).await
 }
 
 /// Run `xwininfo` and let the user click the window they want to share, then