diff --git a/internal/engine/stream_source.go b/internal/engine/stream_source.go
index 98aca74..2dc1d3c 100644
--- a/internal/engine/stream_source.go
+++ b/internal/engine/stream_source.go
@@ -7,7 +7,7 @@ import (
 	"io"
 	"os"
 	"path/filepath"
-	"sync"
+	"strings"
 	"sync/atomic"
 	"time"
 )
@@ -80,11 +80,11 @@ type transcodeSource struct {
 	name     string
 	estimate int64
 
-	mu        sync.Mutex
-	cond      *sync.Cond
+	ctx       context.Context
+	notify    chan struct{} // size grew or final flipped; cap=1, non-blocking send
 	size      atomic.Int64
 	final     atomic.Bool
-	failure   error
+	failure   atomic.Pointer[error]
 	startedAt time.Time
 }
 
@@ -119,8 +119,7 @@ func newTranscodeSource(
 
 	// Spawn ffmpeg directly (not via NewTranscoder pipe) so it writes to
 	// disk in real time. We re-use the rest of TranscodeOpts wiring.
-	cmd := &Transcoder{}
-	cmd, err = startTranscoderToFile(ctx, opts.FFmpegPath, args, cmd)
+	cmd, err := startTranscoderToFile(ctx, opts.FFmpegPath, args, nil)
 	if err != nil {
 		os.Remove(tmpPath)
 		return nil, err
@@ -133,9 +132,10 @@ func newTranscodeSource(
 		cmd:       cmd,
 		name:      displayName,
 		estimate:  estimate,
+		ctx:       ctx,
+		notify:    make(chan struct{}, 1),
 		startedAt: time.Now(),
 	}
-	t.cond = sync.NewCond(&t.mu)
 
 	// Re-open the tmp file for reading; ffmpeg keeps writing to it.
 	rf, err := os.Open(tmpPath)
@@ -151,6 +151,17 @@ func newTranscodeSource(
 	return t, nil
 }
 
+// signalNotify wakes any goroutine blocked in ReadAt. Non-blocking: if a
+// notification is already pending the new event is folded into it (callers
+// always re-check size + final after waking, so a coalesced signal still
+// produces correct behaviour).
+func (t *transcodeSource) signalNotify() {
+	select {
+	case t.notify <- struct{}{}:
+	default:
+	}
+}
+
 // watchSize polls the temp file size every 200 ms and wakes any blocked
 // ReadAt callers once new bytes arrive.
 func (t *transcodeSource) watchSize(ctx context.Context) {
@@ -159,16 +170,12 @@ func (t *transcodeSource) watchSize(ctx context.Context) {
 	for {
 		select {
 		case <-ctx.Done():
-			t.mu.Lock()
-			t.cond.Broadcast()
-			t.mu.Unlock()
+			t.signalNotify()
 			return
 		case <-ticker.C:
 		}
 		if t.final.Load() {
-			t.mu.Lock()
-			t.cond.Broadcast()
-			t.mu.Unlock()
+			t.signalNotify()
 			return
 		}
 		stat, err := os.Stat(t.tmpPath)
@@ -178,91 +185,83 @@ func (t *transcodeSource) watchSize(ctx context.Context) {
 		current := stat.Size()
 		if current > t.size.Load() {
 			t.size.Store(current)
-			t.mu.Lock()
-			t.cond.Broadcast()
-			t.mu.Unlock()
+			t.signalNotify()
 		}
 	}
 }
 
-// watchExit waits for ffmpeg to exit and locks in the final size.
+// watchExit waits for ffmpeg to exit (via Transcoder's single-Wait goroutine)
+// and locks in the final size. A kill triggered by Close() is NOT a failure.
 func (t *transcodeSource) watchExit() {
-	err := t.cmd.cmd.Wait()
-	if err != nil && !isExpectedExit(err) {
-		t.mu.Lock()
-		t.failure = fmt.Errorf("ffmpeg exited: %w (%s)", err, t.cmd.Stderr())
-		t.mu.Unlock()
+	<-t.cmd.Done()
+	err := t.cmd.WaitErr()
+	if err != nil && !t.cmd.IsClosing() {
+		failure := fmt.Errorf("ffmpeg exited: %w (%s)", err, t.cmd.Stderr())
+		t.failure.Store(&failure)
 	}
 	if stat, err := os.Stat(t.tmpPath); err == nil {
 		t.size.Store(stat.Size())
 	}
 	t.final.Store(true)
-	t.mu.Lock()
-	t.cond.Broadcast()
-	t.mu.Unlock()
+	t.signalNotify()
 }
 
-func isExpectedExit(err error) bool {
-	// Killed by Close() — pion DC closed, that's fine.
-	if err == nil {
-		return true
+// loadFailure returns the current failure (or nil) without taking a lock.
+func (t *transcodeSource) loadFailure() error {
+	if p := t.failure.Load(); p != nil {
+		return *p
 	}
-	return false
+	return nil
 }
 
 func (t *transcodeSource) ReadAt(p []byte, off int64) (int, error) {
-	if t.failure != nil {
-		return 0, t.failure
+	if err := t.loadFailure(); err != nil {
+		return 0, err
 	}
-	if int64(len(p)) == 0 {
+	if len(p) == 0 {
 		return 0, nil
 	}
-	deadline := time.Now().Add(readBlockTimeout)
+	if off < 0 {
+		return 0, fmt.Errorf("transcode source: negative offset %d", off)
+	}
+	want := int64(len(p))
 
+	deadline := time.Now().Add(readBlockTimeout)
 	for {
+		if t.final.Load() {
+			break
+		}
 		size := t.size.Load()
-		if off+int64(len(p)) <= size || t.final.Load() {
+		// Overflow-safe form of "off + want <= size":
+		if size >= off && size-off >= want {
 			break
 		}
-		// Need to wait for ffmpeg to write more.
-		t.mu.Lock()
-		// Check again under lock to avoid lost wakeup.
-		size = t.size.Load()
-		if off+int64(len(p)) <= size || t.final.Load() {
-			t.mu.Unlock()
+		remaining := time.Until(deadline)
+		if remaining <= 0 {
 			break
 		}
-		// Wait with timeout via a small sleep loop — sync.Cond doesn't
-		// support timed wait, and a goroutine-per-sleep pattern works fine
-		// for our scale.
-		waited := time.NewTimer(500 * time.Millisecond)
-		done := make(chan struct{})
-		go func() {
-			t.cond.Wait()
-			close(done)
-		}()
-		t.mu.Unlock()
+		wait := 500 * time.Millisecond
+		if remaining < wait {
+			wait = remaining
+		}
 		select {
-		case <-done:
-		case <-waited.C:
-			t.mu.Lock()
-			t.cond.Broadcast() // wake the goroutine so it can return
-			t.mu.Unlock()
-			<-done
-		}
-		if time.Now().After(deadline) {
-			break
+		case <-t.ctx.Done():
+			return 0, t.ctx.Err()
+		case <-t.notify:
+		case <-time.After(wait):
 		}
 	}
 
-	if t.failure != nil {
-		return 0, t.failure
+	if err := t.loadFailure(); err != nil {
+		return 0, err
 	}
 
 	n, err := t.tmpFile.ReadAt(p, off)
-	// On growing file ReadAt returns io.EOF when reading past current size.
-	// Convert to io.ErrUnexpectedEOF only when we actually exceeded the
-	// final size; otherwise return n, nil so the pump sends what we have.
+	// On a growing file ReadAt returns io.EOF when reading past current size.
+	// Translate that into "send what we have, RangeEnd will follow" by
+	// returning (n, nil) so the pump treats the data as a partial chunk and
+	// caller re-requests once more bytes appear. Only true EOF (final=true)
+	// propagates as io.EOF.
 	if err == io.EOF && !t.final.Load() {
 		if n > 0 {
 			return n, nil
@@ -281,23 +280,26 @@ func (t *transcodeSource) EstimatedSize() int64 {
 	return t.estimate
 }
 func (t *transcodeSource) FileName() string {
-	// Keep the original extension stripped — output is always fragmented MP4.
-	base := t.name
-	if i := lastIndexByte(base, '.'); i >= 0 {
-		base = base[:i]
-	}
-	return base + ".mp4"
+	// Output is always fragmented MP4 regardless of source extension.
+	return strings.TrimSuffix(t.name, filepath.Ext(t.name)) + ".mp4"
 }
 func (t *transcodeSource) Transcoded() bool { return true }
 func (t *transcodeSource) Close() error {
-	_ = t.cmd.Close()
+	var errs []error
+	if err := t.cmd.Close(); err != nil {
+		errs = append(errs, err)
+	}
 	if t.tmpFile != nil {
-		_ = t.tmpFile.Close()
+		if err := t.tmpFile.Close(); err != nil {
+			errs = append(errs, err)
+		}
 	}
 	if t.tmpPath != "" {
-		_ = os.Remove(t.tmpPath)
+		if err := os.Remove(t.tmpPath); err != nil && !os.IsNotExist(err) {
+			errs = append(errs, err)
+		}
 	}
-	return nil
+	return errors.Join(errs...)
 }
 
 // estimateOutputSize converts probed bitrate × duration into a byte estimate
@@ -343,12 +345,3 @@ func parseBitrateKbps(s string, fallback int) int {
 	}
 	return v * mult
 }
-
-func lastIndexByte(s string, c byte) int {
-	for i := len(s) - 1; i >= 0; i-- {
-		if s[i] == c {
-			return i
-		}
-	}
-	return -1
-}
diff --git a/internal/engine/transcoder.go b/internal/engine/transcoder.go
index 9f38cd6..1752d6d 100644
--- a/internal/engine/transcoder.go
+++ b/internal/engine/transcoder.go
@@ -35,6 +35,10 @@ type TranscodeOpts struct {
 // One Transcoder == one playback position. A seek beyond the buffered window
 // requires Close()ing this transcoder and starting a new one with a higher
 // StartSeconds (handled in webrtc_stream.go).
+//
+// A single internal goroutine owns cmd.Wait() — never call cmd.Wait()
+// directly from outside (os/exec forbids concurrent Wait callers). Use
+// Done() / WaitErr() instead.
 type Transcoder struct {
 	cmd *exec.Cmd
 	out io.ReadCloser
@@ -42,6 +46,9 @@ type Transcoder struct {
 	mu     sync.Mutex
 	closed bool
 	stderr strings.Builder
+
+	done    chan struct{} // closed once cmd.Wait returns; nil if cmd never started
+	waitErr error         // populated before done is closed; read-only after
 }
 
 // NewTranscoder spawns ffmpeg and returns a Transcoder whose Read() yields
@@ -61,6 +68,7 @@ func NewTranscoder(ctx context.Context, filePath string, opts TranscodeOpts) (*T
 	if err := cmd.Start(); err != nil {
 		return nil, fmt.Errorf("transcoder: start ffmpeg: %w", err)
 	}
+	t.startWaitGoroutine()
 	return t, nil
 }
 
@@ -81,13 +89,43 @@ func startTranscoderToFile(ctx context.Context, ffmpegPath string, args []string
 	if err := cmd.Start(); err != nil {
 		return nil, fmt.Errorf("transcoder: start ffmpeg: %w", err)
 	}
+	t.startWaitGoroutine()
 	return t, nil
 }
 
+// startWaitGoroutine launches the single goroutine that owns cmd.Wait().
+// Idempotent — protected by sync.Once-via-nil-check on done.
+func (t *Transcoder) startWaitGoroutine() {
+	if t.done != nil {
+		return
+	}
+	t.done = make(chan struct{})
+	go func() {
+		t.waitErr = t.cmd.Wait()
+		close(t.done)
+	}()
+}
+
+// Done returns a channel that closes when ffmpeg exits. Returns nil for a
+// Transcoder whose cmd never started.
+func (t *Transcoder) Done() <-chan struct{} { return t.done }
+
+// WaitErr blocks until ffmpeg exits and returns the wait error. Safe to
+// call concurrently from multiple goroutines.
+func (t *Transcoder) WaitErr() error {
+	if t.done == nil {
+		return nil
+	}
+	<-t.done
+	return t.waitErr
+}
+
 // Read implements io.Reader.
 func (t *Transcoder) Read(p []byte) (int, error) { return t.out.Read(p) }
 
 // Close kills the child process if still running and waits up to 2s for exit.
+// IsClosing reports true after Close has been invoked — used by streamSource
+// to distinguish a kill-by-Close from a genuine ffmpeg crash.
 func (t *Transcoder) Close() error {
 	t.mu.Lock()
 	if t.closed {
@@ -106,19 +144,26 @@ func (t *Transcoder) Close() error {
 	if t.cmd != nil && t.cmd.Process != nil {
 		_ = t.cmd.Process.Kill()
 	}
-	if t.cmd == nil {
+	if t.done == nil {
 		return nil
 	}
-	done := make(chan error, 1)
-	go func() { done <- t.cmd.Wait() }()
 	select {
-	case <-done:
+	case <-t.done:
 	case <-time.After(2 * time.Second):
 		// Process refused to die — leak it; the OS will clean up on exit.
 	}
 	return nil
 }
 
+// IsClosing reports whether Close has been invoked. Cheap atomic-ish check
+// for callers that want to distinguish a kill-by-Close exit from a real
+// ffmpeg failure when reading WaitErr.
+func (t *Transcoder) IsClosing() bool {
+	t.mu.Lock()
+	defer t.mu.Unlock()
+	return t.closed
+}
+
 // Stderr returns the accumulated ffmpeg stderr so far. Useful for surfacing
 // failure reasons in logs after Close().
 func (t *Transcoder) Stderr() string {
@@ -185,27 +230,47 @@ func buildFFmpegArgs(filePath string, opts TranscodeOpts) []string {
 		if videoCodec == "libx264" {
 			args = append(args, "-preset", coalesce(opts.Preset, "veryfast"))
 		}
+		// Force the broadest browser-compatible h264 profile. `high` (libx264
+		// default) makes Chrome try its hardware decoder path first, which
+		// can fail with "VaapiWrapper: failed initializing" on Linux boxes
+		// where VA-API isn't fully wired up. `main` keeps a clean software
+		// decode fallback on every desktop + mobile platform.
+		args = append(args, "-profile:v", "main", "-level:v", "4.0")
 		args = append(args, "-b:v", coalesce(opts.VideoBitrate, "5M"))
+		// Filter chain:
+		//   1. scale (optional) — cap height + force even width.
+		//   2. format=yuv420p — drop 10-bit + reset pix_fmt to 8-bit before
+		//      libx264 (which refuses 10-bit unless built with --bit-depth=10).
+		//   3. setparams — REWRITE the color metadata in the output stream's
+		//      VUI/SEI without touching pixels. This is what makes HDR HEVC
+		//      sources (color_primaries=bt2020, color_transfer=arib-std-b67)
+		//      decodeable in browsers that reject anything but Rec.709. We
+		//      can't actually tonemap without libzimg/zscale (most ffmpeg
+		//      builds — including ours — ship without it), so colours look
+		//      desaturated on HDR sources, but the file plays. SDR sources
+		//      already match these params and are unaffected.
+		var filterChain string
 		if opts.MaxHeight > 0 {
-			// `-2:H` scales to height H, derives width preserving aspect ratio,
-			// and rounds to a multiple of 2 (libx264 refuses odd dimensions).
-			// `force_original_aspect_ratio=decrease` keeps shorter sources
-			// untouched instead of upscaling. `pix_fmt yuv420p` keeps 10-bit
-			// HEVC sources playable in browsers (8-bit only).
-			args = append(args,
-				"-vf",
-				fmt.Sprintf("scale=-2:%d:force_original_aspect_ratio=decrease", opts.MaxHeight),
-				"-pix_fmt", "yuv420p",
+			filterChain = fmt.Sprintf(
+				"scale=-2:%d:force_original_aspect_ratio=decrease,format=yuv420p,setparams=colorspace=bt709:color_trc=bt709:color_primaries=bt709:range=tv",
+				opts.MaxHeight,
 			)
 		} else {
-			args = append(args, "-pix_fmt", "yuv420p")
+			filterChain = "format=yuv420p,setparams=colorspace=bt709:color_trc=bt709:color_primaries=bt709:range=tv"
 		}
+		args = append(args, "-vf", filterChain)
 		args = append(args, "-c:a", "aac", "-b:a", coalesce(opts.AudioBitrate, "192k"))
 	}
 
 	// Common output flags — fragmented MP4 to a single pipe.
+	// NO faststart: that flag rewrites the moov atom to the front of the
+	// file as a SECOND pass after encoding finishes, which means the
+	// browser never sees a moov until ffmpeg exits. For live transcoding
+	// we need empty_moov (write a placeholder up front) so MSE can start
+	// decoding the very first fragment. faststart is only safe for
+	// already-finished files.
 	args = append(args,
-		"-movflags", "frag_keyframe+empty_moov+default_base_moof+faststart",
+		"-movflags", "frag_keyframe+empty_moov+default_base_moof",
 		"-f", "mp4",
 		"pipe:1",
 	)
diff --git a/internal/engine/webrtc_stream.go b/internal/engine/webrtc_stream.go
index 691d5a3..e3c6378 100644
--- a/internal/engine/webrtc_stream.go
+++ b/internal/engine/webrtc_stream.go
@@ -145,7 +145,7 @@ func buildStreamSource(
 	log StreamLogger,
 ) (streamSource, error) {
 	tc := cfg.Transcode
-	cap := resolveQualityCap(cfg.Quality)
+	qcap := resolveQualityCap(cfg.Quality)
 
 	if tc.Disabled || tc.FFmpegPath == "" || tc.FFprobePath == "" {
 		return newDiskFileSource(abs)
@@ -160,14 +160,10 @@ func buildStreamSource(
 
 	// Quality cap can promote a passthrough/remux decision into a full video
 	// transcode when the source resolution exceeds the requested cap.
-	forcedByQuality := false
-	if cap.MaxHeight > 0 && probe.Height > 0 && probe.Height > cap.MaxHeight {
-		if action != ActionTranscodeVideo {
-			log.Infof("[wrtc %s] quality=%s caps height %d→%d — forcing video transcode",
-				agent.ShortID(cfg.SessionID), cfg.Quality, probe.Height, cap.MaxHeight)
-			action = ActionTranscodeVideo
-			forcedByQuality = true
-		}
+	if qcap.MaxHeight > 0 && probe.Height > 0 && probe.Height > qcap.MaxHeight && action != ActionTranscodeVideo {
+		log.Infof("[wrtc %s] quality=%s caps height %d→%d — forcing video transcode",
+			agent.ShortID(cfg.SessionID), cfg.Quality, probe.Height, qcap.MaxHeight)
+		action = ActionTranscodeVideo
 	}
 
 	if action == ActionPassthrough {
@@ -178,15 +174,14 @@ func buildStreamSource(
 
 	log.Infof("[wrtc %s] transcoding %s/%s/%s → h264+aac (%s, quality=%s)",
 		agent.ShortID(cfg.SessionID), probe.Container, probe.VideoCodec, probe.AudioCodec,
-		action, coalesceLabel(cfg.Quality))
+		action, coalesce(cfg.Quality, "default"))
 
 	maxHeight := tc.MaxHeight
 	videoBitrate := tc.VideoBitrate
-	if cap.MaxHeight > 0 {
-		maxHeight = cap.MaxHeight
-		videoBitrate = cap.VideoBitrate
+	if qcap.MaxHeight > 0 {
+		maxHeight = qcap.MaxHeight
+		videoBitrate = qcap.VideoBitrate
 	}
-	_ = forcedByQuality // reserved for future telemetry
 
 	opts := TranscodeOpts{
 		Action:       action,
@@ -200,13 +195,6 @@ func buildStreamSource(
 	return newTranscodeSource(ctx, abs, probe, action, opts, displayName)
 }
 
-func coalesceLabel(s string) string {
-	if s == "" {
-		return "default"
-	}
-	return s
-}
-
 // RunWebRTCStream blocks until the session ends — either the DataChannel
 // closes, the peer connection drops, or ctx is cancelled. Always returns a
 // non-nil error explaining the termination reason.
@@ -520,6 +508,13 @@ func (p *dataChannelPump) onOpen() {
 	// ffmpeg writes more bytes; the estimate just bootstraps the UI.
 	announceSize := p.source.EstimatedSize()
 	transcoding := p.source.Transcoded()
+	// Browsers refuse to start playback when Content-Length is 0. If we don't
+	// have a duration estimate (e.g. ffprobe couldn't tag the source), declare
+	// a large sentinel so the browser issues range requests; the Transcoding
+	// flag tells it the value is provisional.
+	if transcoding && announceSize <= 0 {
+		announceSize = math.MaxInt64
+	}
 	// Seekable=true even for transcoded sources because we read from a tmp
 	// file (random access). Seek backwards just works; seek forward beyond
 	// what ffmpeg has produced will block briefly inside ReadAt.
@@ -633,22 +628,35 @@ func (p *dataChannelPump) serveRange(streamID uint32, req wire.RangeReqPayload)
 	if req.Length > math.MaxInt64 {
 		want = 0 // treat absurd length as "remainder of file"
 	}
-	// "Remainder" target: prefer current known size, fall back to estimate
-	// for transcoded streams so the browser can keep scrolling forward as
-	// ffmpeg produces output.
-	knownEnd := currentSize
-	if p.source.Final() {
-		knownEnd = finalSize
+	// Cap by *final* size, not currentSize. For a still-transcoding stream
+	// currentSize grows over time and ReadAt below already blocks until
+	// ffmpeg produces the requested bytes (with a deadline). If we cap
+	// `want` by currentSize here we'll send an empty RangeEnd whenever the
+	// browser asks for bytes faster than ffmpeg writes them — which is
+	// always true on the first few seconds — and the browser then aborts
+	// playback with "Format error".
+	cap := finalSize
+	if !p.source.Final() && cap < int64(req.Offset)+1 {
+		// Estimate too small: serve as much as the browser asked for and
+		// let ReadAt block.
+		cap = int64(req.Offset) + want
 	}
-	if knownEnd < int64(req.Offset) {
-		knownEnd = int64(req.Offset)
+	if int64(req.Offset) >= cap && p.source.Final() {
+		// Past true end of a finished file.
+		p.sendRangeEnd(streamID, 0)
+		return
+	}
+	remaining := cap - int64(req.Offset)
+	if remaining < 0 {
+		remaining = 0
 	}
-	remaining := knownEnd - int64(req.Offset)
 	if want <= 0 || want > remaining {
 		want = remaining
 	}
+	p.log.Infof("dc: range_req sid=%d offset=%d wantReq=%d wantServe=%d currentSize=%d final=%v",
+		streamID, req.Offset, req.Length, want, currentSize, p.source.Final())
 	if want <= 0 {
-		// Nothing to serve right now (transcoder hasn't reached this offset).
+		// Only happens for a finished file when offset is at/past EOF.
 		p.sendRangeEnd(streamID, 0)
 		return
 	}