[Playback] Enabling PGS subtitles triggers unexpected forced transcode and frame drops

[userobvixus]

Moonfin version

2.1.0

Platform

Android

OS version

IOS 26.5

Device model

iPhone 12 Pro Max

Input method

Touch

Server type and version

Jellyfin 10.11.11

Content type

Movie

Media details (as much as you know, check the info button at the bottom right of the player)

• File: Game of Thrones - S05E07 - The Gift Bluray-2160p MULTI [x265]-QTZ.mkv
• Container: MKV
• Resolution: 3840x2160 @ 24fps
• Video: HEVC (H.265) Main 10@L150
• Subtitles involved: PGS (Bitmap)

Playback path

Transcode

Symptoms (check all that apply)

• Black screen / endless spinner
• Audio only (no video)
• Stuttering / frame drops
• A/V sync issues
• Seeking/scrubbing broken
• Subtitles missing or incorrect
• Unexpected forced transcode
• App crashes / exits

Steps to reproduce

1. Play a 4K HEVC media file with multiple subtitle formats available.
2. Ensure the playback is in "Direct Play" (e.g., using SRT subtitles or no subtitles).
3. Open the subtitle menu and select a PGS (bitmap) subtitle track.
4. Observe the playback info: the session immediately switches from "Direct Play" to "Transcode".
5. Observe the playback quality: the video becomes laggy, behaving as if the framerate has significantly dropped compared to the Direct Play session.

Frequency

Always

Logs / errors (optional)

Since media_kit (libmpv) is supposed to handle PGS bitmap subtitles natively, the forced transcode request to the server seems unexpected. The resulting server-side burn-in process for the 4K file introduces the stuttering/frame drops observed on the client.