diff --git a/movie_editor/frontend/player.js b/movie_editor/frontend/player.js
index 6a1f8b31..4c307bb0 100644
--- a/movie_editor/frontend/player.js
+++ b/movie_editor/frontend/player.js
@@ -579,33 +579,87 @@
     return v;
   }
 
+  // Materializing a <video> per clip URL for the WHOLE timeline breaks big (montage)
+  // timelines: Chrome refuses new media players past ~75 per page, and every element's
+  // preload=metadata fetch of a per-clip preview segment (each a server-side ffmpeg
+  // encode) fights over the browser's ~6 per-origin connections on a tunneled instance —
+  // so clips in the back half of the timeline sit "loading" forever while the front half
+  // plays. Only clips in a window around the playhead get elements; the window keeps cut
+  // boundaries instant (upcoming clips are materialized ahead) while the element count
+  // stays flat no matter how long the timeline is.
+  const POOL_BEHIND = 2, POOL_AHEAD = 8, POOL_MAX = 16;
+
+  function _windowUrls() {
+    const urls = new Set();
+    let idx = _currentClip ? _clips.indexOf(_currentClip) : -1;
+    if (idx < 0) {
+      const at = _clipAt(_phSec);
+      idx = at ? _clips.indexOf(at) : -1;
+    }
+    if (idx < 0) idx = _clips.findIndex(_hasPlayableClipMedia);
+    if (idx >= 0) {
+      for (let i = Math.max(0, idx - POOL_BEHIND); i < _clips.length && i <= idx + POOL_AHEAD; i++) {
+        const c = _clips[i];
+        if (!c || c.pending || c.blank) continue;
+        const u = _clipUrl(c);
+        if (u) urls.add(u);
+      }
+    }
+    return urls;
+  }
+
+  function _releasePooledVideo(u, v) {
+    pool.delete(u);
+    if (_active === v) _active = null;
+    v.pause();
+    clearTimeout(v._pmRetryTimer);
+    // Release the socket BEFORE detaching: a detached media element that still has a
+    // src holds its per-origin connection until GC (media-connection-pool class).
+    try { v.removeAttribute("src"); v.load(); } catch (_) {}
+    v._pmFx?.viewport.remove();
+  }
+
+  // Materialize window members that are missing; evict far elements once the pool
+  // outgrows POOL_MAX (the slack keeps recently-visited clips warm for back-and-forth
+  // scrubbing instead of re-fetching on every jump). Runs at every playhead-affecting
+  // step via _updatePreloadHints, and is idempotent/cheap when nothing changed.
+  function _syncPoolWindow() {
+    const want = _windowUrls();
+    for (const u of want) {
+      if (pool.has(u)) continue;
+      const clip = _clips.find((c) => _clipUrl(c) === u);
+      if (clip) _ensureVideo(u, clip);
+    }
+    if (pool.size <= POOL_MAX) return;
+    for (const [u, v] of [...pool]) {  // Map order = insertion order ⇒ oldest first
+      if (pool.size <= POOL_MAX) break;
+      if (want.has(u) || v === _active) continue;
+      _releasePooledVideo(u, v);
+    }
+  }
+
   function _syncPool() {
     _urlClips.clear();
-    const urls = new Set(_clips.filter((c) => c.media || c.binUrl || c.streamUrl).map((c) => _clipUrl(c)));
-    urls.forEach((u) => {
+    // Timeline rebuild: drop elements whose URL no longer exists on any clip at all
+    // (deleted/regenerated clips) regardless of the window.
+    const live = new Set(_clips.filter((c) => c.media || c.binUrl || c.streamUrl).map((c) => _clipUrl(c)));
+    for (const [u, v] of [...pool]) {
+      if (live.has(u)) continue;
+      _releasePooledVideo(u, v);
+    }
+    _syncPoolWindow();
+    for (const [u, v] of pool) {
       const clip = _clips.find((c) => _clipUrl(c) === u);
-      const v = _ensureVideo(u, clip);
+      if (clip) _registerUrlClip(u, clip);
       // Clip rebuild (e.g. a run just completed): give exhausted-but-errored elements a
       // fresh recovery attempt — the failure was likely the server being busy back then.
       if (v.error && !v._pmRetryTimer) { v._pmRetries = 0; _scheduleVideoRetry(u, v); }
-    });
-    for (const [u, v] of [...pool]) {
-      if (urls.has(u)) continue;
-      v.pause();
-      clearTimeout(v._pmRetryTimer);
-      v._pmFx?.viewport.remove();
-      pool.delete(u);
-      if (_active === v) { _active = null; }
     }
   }
 
   function _resetPool() {
     _stopTick();
-    for (const [, v] of pool) {
-      v.pause();
-      clearTimeout(v._pmRetryTimer);
-      v._pmFx?.viewport.remove();
-    }
+    for (const [u, v] of [...pool]) _releasePooledVideo(u, v);
     pool.clear(); _active = null; _currentClip = null; _seekPending = null; _playPending = false;
     _urlClips.clear();
     _clipBoundaryToken = 0;
@@ -735,6 +789,7 @@
   // stay instant); everything else idles at metadata so the pool doesn't saturate the
   // browser's per-origin connections. A hint change never interrupts an in-flight load.
   function _updatePreloadHints() {
+    _syncPoolWindow();  // playhead moved — materialize upcoming clips, evict far ones
     if (!pool.size) return;
     const hot = new Set();
     const add = (c) => {
diff --git a/movie_editor/frontend/timeline.js b/movie_editor/frontend/timeline.js
index 9e8ffffb..d420816f 100644
--- a/movie_editor/frontend/timeline.js
+++ b/movie_editor/frontend/timeline.js
@@ -201,6 +201,23 @@
     });
   }
 
+  // Filmstrip probes run through a small shared queue: painting a big (montage) timeline
+  // used to start one preload=auto <video> per clip AT ONCE — each of them a server-side
+  // ffmpeg segment encode — saturating both the browser's ~6 per-origin connections and
+  // the server, and starving the player pool (clips stuck "loading" on tunneled
+  // instances). Two at a time keeps thumbnails flowing without wedging playback.
+  const _fsQueue = [];
+  let _fsActive = 0;
+  const FS_CONCURRENCY = 2;
+  function _fsPump() {
+    while (_fsActive < FS_CONCURRENCY && _fsQueue.length) {
+      const job = _fsQueue.shift();
+      _fsActive++;
+      job().catch(() => {}).then(() => { _fsActive--; _fsPump(); });
+    }
+  }
+  function _fsEnqueue(job) { _fsQueue.push(job); _fsPump(); }
+
   function appendFilmstrip(clip, st, scene, widthPx) {
     if (S.isVideoClip && S.isVideoClip(scene)) return;
     if (!hasRender(st, scene.id)) return;
@@ -220,47 +237,60 @@
       ? API.previewSegmentUrl(st.project.id, scene.id, { media: r.media, renderIn: r.inSec || 0, dur: durSec })
       : null;
     const url = segUrl || API.resultUrl(st.project.id, r.media);
-    const vid = document.createElement("video");
-    vid.muted = true; vid.preload = "auto"; vid.src = url;
-    const captureAt = (time) => new Promise((resolve) => {
-      if (!strip.isConnected) { resolve(); return; }
-      const cell = el("div", "clip-fs-cell");
-      const c = document.createElement("canvas");
-      c.width = 48; c.height = 27;
-      const ctx = c.getContext("2d");
-      const thumb = el("img", "clip-fs-thumb");
-      thumb.alt = "";
-      const done = () => {
-        if (!strip.isConnected) { resolve(); return; }
-        try { ctx.drawImage(vid, 0, 0, 48, 27); thumb.src = c.toDataURL("image/jpeg", 0.55); } catch (_) {}
-        cell.append(thumb);
-        strip.append(cell);
-        resolve();
+    _fsEnqueue(() => new Promise((finish) => {
+      if (!strip.isConnected) { finish(); return; }  // re-rendered before its turn came up
+      const vid = document.createElement("video");
+      vid.muted = true; vid.preload = "auto"; vid.src = url;
+      // Free the media connection once the strip is captured (or abandoned) — a detached
+      // <video> keeps its socket until GC, and those add up against Chrome's 6-per-origin
+      // cap. Also settles the queue job; the guard timer keeps a wedged load (server busy,
+      // dead tunnel) from clogging a queue slot forever.
+      let settled = false;
+      const release = () => {
+        if (settled) return;
+        settled = true;
+        clearTimeout(guard);
+        try { vid.removeAttribute("src"); vid.load(); } catch (_) {}
+        finish();
       };
-      vid.onseeked = done;
-      const dur = vid.duration || durSec;
-      vid.currentTime = Math.max(0, Math.min(time, dur - 0.01));
-    });
-    // Free the media connection once the strip is captured (or abandoned) — a detached
-    // <video> keeps its socket until GC, and those add up against Chrome's 6-per-origin cap.
-    const release = () => { try { vid.removeAttribute("src"); vid.load(); } catch (_) {} };
-    vid.onerror = release;
-    vid.onloadeddata = async () => {
-      if (!strip.isConnected) { release(); return; }
-      const dur = vid.duration || durSec;
-      // A segment starts at 0 (it's already trimmed to this scene); the raw-file fallback
-      // still needs the absolute in-point.
-      const inSec = segUrl ? 0 : (r.inSec || 0) + (scene.source_in || 0);
-      try {
-        for (let i = 0; i < n; i++) {
-          if (!strip.isConnected) return;
-          const t = inSec + (dur * (i + 0.5) / n);
-          await captureAt(t);
+      const guard = setTimeout(release, 20000);
+      const captureAt = (time) => new Promise((resolve) => {
+        if (!strip.isConnected) { resolve(); return; }
+        const cell = el("div", "clip-fs-cell");
+        const c = document.createElement("canvas");
+        c.width = 48; c.height = 27;
+        const ctx = c.getContext("2d");
+        const thumb = el("img", "clip-fs-thumb");
+        thumb.alt = "";
+        const done = () => {
+          if (!strip.isConnected) { resolve(); return; }
+          try { ctx.drawImage(vid, 0, 0, 48, 27); thumb.src = c.toDataURL("image/jpeg", 0.55); } catch (_) {}
+          cell.append(thumb);
+          strip.append(cell);
+          resolve();
+        };
+        vid.onseeked = done;
+        const dur = vid.duration || durSec;
+        vid.currentTime = Math.max(0, Math.min(time, dur - 0.01));
+      });
+      vid.onerror = release;
+      vid.onloadeddata = async () => {
+        if (!strip.isConnected) { release(); return; }
+        const dur = vid.duration || durSec;
+        // A segment starts at 0 (it's already trimmed to this scene); the raw-file fallback
+        // still needs the absolute in-point.
+        const inSec = segUrl ? 0 : (r.inSec || 0) + (scene.source_in || 0);
+        try {
+          for (let i = 0; i < n; i++) {
+            if (!strip.isConnected) return;
+            const t = inSec + (dur * (i + 0.5) / n);
+            await captureAt(t);
+          }
+        } finally {
+          release();
         }
-      } finally {
-        release();
-      }
-    };
+      };
+    }));
   }
 
   const hasRender = (st, sceneId) => !!(st.sceneRenders && st.sceneRenders[sceneId] && st.sceneRenders[sceneId].media);
diff --git a/movie_editor/server.py b/movie_editor/server.py
index 49d644d3..c6a48934 100644
--- a/movie_editor/server.py
+++ b/movie_editor/server.py
@@ -209,6 +209,14 @@ def _ffmpeg_trim_clip(src_path: str, out_path: str, inn, dur, fast: bool = False
         raise RuntimeError((proc.stderr or "ffmpeg failed")[-1000:])
 
 
+_ISOBMFF_EXTS = (".mp4", ".m4v", ".mov")  # containers where moov/mdat atom order applies
+
+
+def _is_isobmff(path: str) -> bool:
+    import os
+    return os.path.splitext(path)[1].lower() in _ISOBMFF_EXTS
+
+
 def _moov_position(path: str) -> str:
     """Where the MP4's moov atom sits: "front" (before mdat — browsers can decode from a
     progressive/Range fetch), "end" (after mdat — ComfyUI's own VHS_VideoCombine output; a
@@ -343,8 +351,9 @@ _preview_segment_cache: dict[str, str] = {}
 _preview_segment_locks: dict = {}  # cache key -> asyncio.Lock (dedupe concurrent encodes)
 
 _faststart_cache: dict[str, tuple] = {}  # source path -> (source sig, remuxed faststart path)
-_faststart_raw_ok: dict[str, tuple] = {}  # source path -> sig a stable file failed to remux at (serve raw, don't re-run ffmpeg per request)
+_faststart_failed: dict[str, tuple] = {}  # source path -> (sig, monotonic ts of last failed remux attempt)
 _faststart_locks: dict = {}  # source path -> asyncio.Lock (dedupe concurrent remuxes)
+_FASTSTART_RETRY_SEC = 30.0  # min gap between remux re-attempts on a file that failed to remux
 
 
 def _preview_segment_cache_key(pid: str, scene_id: str, clip: dict) -> str:
@@ -1831,11 +1840,18 @@ if web is not None and PromptServer is not None:
         offsets (PIPELINE decode errors) or fails If-Range and restarts the clip from 0.
         Caches are keyed on the source's (mtime, size) signature and the remux output name is
         derived from path+signature, so a rewritten source can't serve a stale copy and two
-        sources sharing a basename can't overwrite each other's remux mid-stream."""
+        sources sharing a basename can't overwrite each other's remux mid-stream.
+
+        A moov-at-end file must NEVER be served raw, whatever goes wrong here: deep seeks
+        into it are undecodable in the browser (Firefox: NS_ERROR_DOM_MEDIA_DECODE_ERR /
+        AppleVTDecoder reference-missing), and the element's cache-busted retries then fail
+        identically forever. A remux failure answers 502 (rate-limited re-attempts) instead
+        of falling back to raw bytes — an honest "Preview unavailable" beats undecodable video."""
         import asyncio
         import hashlib
         import os
-        if os.path.splitext(src_path)[1].lower() != ".mp4":
+        import time
+        if not _is_isobmff(src_path):
             return src_path
         try:
             sig = _file_sig(src_path)
@@ -1861,15 +1877,21 @@ if web is not None and PromptServer is not None:
                 # will change under this URL. 503 lets the player's retry loop try again.
                 raise web.HTTPServiceUnavailable(
                     reason="Video file is still being written.", headers={"Retry-After": "2"})
-            if _faststart_raw_ok.get(src_path) == sig:
-                return src_path
+            failed = _faststart_failed.get(src_path)
+            if failed and failed[0] == sig and time.monotonic() - failed[1] < _FASTSTART_RETRY_SEC:
+                raise web.HTTPBadGateway(
+                    reason="Video could not be prepared for playback (remux failed).")
             try:
                 import folder_paths
                 tempdir = folder_paths.get_temp_directory()
             except Exception:
-                return src_path
+                # No temp dir means no seek-safe copy can be made; raw moov-at-end bytes
+                # would be undecodable, so tell the player to retry rather than serve them.
+                raise web.HTTPServiceUnavailable(
+                    reason="Temp directory unavailable.", headers={"Retry-After": "5"})
             digest = hashlib.md5(f"{src_path}|{sig[0]}|{sig[1]}".encode()).hexdigest()[:16]
-            out_path = os.path.join(tempdir, f"funpack_faststart_{digest}.mp4")
+            ext = os.path.splitext(src_path)[1].lower()
+            out_path = os.path.join(tempdir, f"funpack_faststart_{digest}{ext}")
             err = None
             try:
                 # to_thread: same event-loop-freeze hazard as preview-segment encodes below —
@@ -1885,6 +1907,7 @@ if web is not None and PromptServer is not None:
                 sig_now = None  # deleted under us — retry will 404 definitively
             if ok and sig_now == sig:
                 _faststart_cache[src_path] = (sig, out_path)
+                _faststart_failed.pop(src_path, None)
                 return out_path
             try:
                 os.remove(out_path)
@@ -1895,8 +1918,9 @@ if web is not None and PromptServer is not None:
                 raise web.HTTPServiceUnavailable(
                     reason="Video file is still being written.", headers={"Retry-After": "2"})
             print(f"[FunPack] faststart remux failed for {src_path}: {err}")
-            _faststart_raw_ok[src_path] = sig  # stable but un-remuxable: serve raw, once
-            return src_path
+            _faststart_failed[src_path] = (sig, time.monotonic())
+            raise web.HTTPBadGateway(
+                reason="Video could not be prepared for playback (remux failed).")
 
     @routes.get(UI_PREFIX + "/api/projects/{pid}/result")
     async def _result(req):
@@ -1913,7 +1937,7 @@ if web is not None and PromptServer is not None:
             # Mirror the GET path's mid-write gate: a moov-less mp4 would 503 on GET, so a
             # HEAD claiming 200 would lie — the player probes HEAD after a media error to
             # tell "render still being written" (show "processing") apart from a dead file.
-            if os.path.splitext(path)[1].lower() == ".mp4" and _moov_position(path) == "none":
+            if _is_isobmff(path) and _moov_position(path) == "none":
                 raise web.HTTPServiceUnavailable(
                     reason="Video file is still being written.", headers={"Retry-After": "2"})
             return web.Response()
@@ -1936,6 +1960,31 @@ if web is not None and PromptServer is not None:
             )
         except Exception as e:  # noqa: BLE001
             raise web.HTTPBadGateway(reason=f"could not fetch result: {e}")
+        fname = req.query.get("filename", "")
+        if _is_isobmff(fname):
+            # A buffered full-body mp4/mov response has no Range support and, for a raw
+            # ComfyUI render, moov at the end — both make seeking undecodable in the
+            # browser. Spool the fetched bytes to temp and serve through the same
+            # remux-and-FileResponse path as resolver hits.
+            import hashlib
+            try:
+                import folder_paths
+                tempdir = folder_paths.get_temp_directory()
+            except Exception:
+                raise web.HTTPServiceUnavailable(
+                    reason="Temp directory unavailable.", headers={"Retry-After": "5"})
+            ext = os.path.splitext(fname)[1].lower()
+            digest = hashlib.md5(
+                f"{fname}|{req.query.get('subfolder', '')}|"
+                f"{req.query.get('type', 'output')}|{len(data)}".encode()).hexdigest()[:16]
+            spool = os.path.join(tempdir, f"funpack_viewspool_{digest}{ext}")
+            if not os.path.isfile(spool) or os.path.getsize(spool) != len(data):
+                tmp = spool + ".part"
+                with open(tmp, "wb") as f:
+                    f.write(data)
+                os.replace(tmp, spool)  # atomic: no reader ever sees a partial spool
+            path = await _faststart_path(spool)
+            return web.FileResponse(path, headers={"Content-Type": ctype.split(";")[0]})
         return web.Response(body=data, content_type=ctype.split(";")[0])
 
     # --- API: final render (async job — ffmpeg can exceed Cloudflare tunnel timeouts) ---
@@ -1995,7 +2044,7 @@ if web is not None and PromptServer is not None:
             return web.json_response({"detail": str(e)}, status=400)
         except OSError as e:
             return web.json_response({"detail": str(e)}, status=503)
-        if os.path.splitext(src_path)[1].lower() == ".mp4" and _moov_position(src_path) == "none":
+        if _is_isobmff(src_path) and _moov_position(src_path) == "none":
             # No moov ⇒ still being written (VHS finalizes it last) or an aborted save:
             # ffmpeg can't trim it and the segment would come out truncated/undecodable.
             return web.json_response(