diff --git a/src/media_processor.py b/src/media_processor.py
index 09330b8..5349f6c 100644
--- a/src/media_processor.py
+++ b/src/media_processor.py
@@ -172,18 +172,24 @@ class MediaProcessor:
 
     async def _apply_faststart(self, path: Path) -> Path:
         """Moves the moov atom to the front with a fast, lossless remux (-c copy - no
-        re-encode, just repositions the index). Needed because muxing a separate
-        video+audio stream (see _select_format's "+bestaudio" case) leaves the moov atom
-        at the end by default: confirmed on a real post where the result played audio
-        with a black video, because streaming players (notably Telegram's mobile clients)
-        can't render video without seeking to the index first. Falls back to the original
-        file - rather than dropping the video - if ffmpeg fails for any reason."""
+        re-encode, just repositions the index and timestamps). Needed because muxing a
+        separate video+audio stream (see _select_format's "+bestaudio" case) leaves the
+        moov atom at the end by default, AND leaves the video track starting at a
+        slightly negative DTS (its first frame is a B-frame reordered before the
+        keyframe's PTS) - confirmed on a real post where the result played audio with a
+        black video on Telegram's mobile app in fullscreen (but rendered fine in PiP,
+        which apparently tolerates it - a strong hint the hardware decoder path is what
+        chokes on it) while desktop played it fine either way. -avoid_negative_ts
+        make_zero fixes the second half; without it, moov-only faststart wasn't enough.
+        Falls back to the original file - rather than dropping the video - if ffmpeg
+        fails for any reason."""
         fixed_path = path.with_name(f"{path.stem}_fs.mp4")
         await register_active_path(fixed_path)
         proc = None
         try:
             proc = await asyncio.create_subprocess_exec(
-                "ffmpeg", "-y", "-i", str(path), "-c", "copy", "-movflags", "+faststart",
+                "ffmpeg", "-y", "-i", str(path), "-c", "copy",
+                "-avoid_negative_ts", "make_zero", "-movflags", "+faststart",
                 str(fixed_path),
                 stdout=asyncio.subprocess.DEVNULL, stderr=asyncio.subprocess.DEVNULL,
             )