MediaPlayer: Seek for integrated fMP4 file

[cathiecode]

Summary

This pull request enables video seeking on videos from certain websites that stream Integrated fMP4 files.

Required checks

All boxes below must be ticked before this PR can merge. If a check is genuinely N/A, tick it anyway and explain under Notes.

• Tested — I built and ran this locally. The change works in the editor and (where relevant) in a built player.
• Transform access is combined and limited — In hot paths, transform reads/writes go through TransformAccessArray or are otherwise batched. I have not added per-frame transform.position / transform.rotation / transform.localPosition calls inside loops. Whenever I need both position and rotation, I use the combined APIs — SetPositionAndRotation / SetLocalPositionAndRotation for writes, GetPositionAndRotation / GetLocalPositionAndRotation for reads — instead of two separate property accesses; the combined call does one local-to-world matrix traversal instead of two.
• Addressables used for asset/memory loading — Any new asset loads go through Addressables. No new Resources.Load, no direct asset references that pull large content into memory on scene load.
• No new GetComponent / AddComponent where avoidable — Where unavoidable, the result is cached on a field, and any GetComponent<T> is replaced with TryGetComponent<T>(out var x) — bare GetComponent will be denied. TryGetComponent is the modern API (Unity 2019.2+) and skips the Editor-only GC allocation GetComponent causes when a component is missing: Unity wraps the null return in a managed "fake null" object so its overloaded == operator can still detect destroyed C++ objects, and constructing that wrapper allocates; TryGetComponent returns a bool plus out parameter and never builds the wrapper. None of these calls run inside Update, LateUpdate, FixedUpdate, jobs, or other per-frame code paths.
• Per-frame work is scheduled through BasisEventDriver — Any new per-frame work hooks into BasisEventDriver rather than adding standalone Update / LateUpdate / FixedUpdate callbacks on a MonoBehaviour.
• Anything added to BasisEventDriver is bulletproof, or guarded by try/catch — BasisEventDriver runs the single per-frame tick that drives the whole framework (network apply, local player sim, blendshapes, JigglePhysics, nameplates, and more) as one sequential chain. An unhandled exception anywhere in that chain aborts the rest of the tick, so every step after the throwing one is silently skipped for that frame. New work added to the driver must either be guaranteed not to throw, or be wrapped in a try/catch that contains the failure and surfaces it through BasisDebug — logged once / rate-limited, never every frame (see the existing HVRBasisBuiltInAddresses.Simulate() guard for the pattern). Expect this to be scrutinized closely in review.
• Considered jobification — I asked whether this work can be moved to a Unity Job (Burst-compiled where possible). If it can, it is. If it cannot, the reason is in Notes.
• No needless { get; set; } properties or access lockdowns — Public fields are fine; Basis is meant to be read and modified freely, so don't wall things off private/internal without a real reason. Don't wrap a field in { get; set; } when the accessors do nothing — property accessors have a real performance cost vs direct field access, and the lead maintainer prefers plain fields (or a method / setter-only property when only the setter needs logic) over a noop-getter pair. For .Instance singletons, callers reassigning Type.Instance is allowed; if that would break your code, log a warning or throw — don't block the assignment. Locking down access is not your call.
• Camera access goes through BasisLocalCameraDriver — Code that needs the local camera (transform, projection, rig data, etc.) pulls it from BasisLocalCameraDriver rather than looking one up itself. Don't roll a separate camera discovery path.
• Logging uses BasisDebug — All new logging calls go through BasisDebug.Log / BasisDebug.LogWarning / BasisDebug.LogError (with an appropriate LogTag) instead of UnityEngine.Debug.Log / Debug.LogWarning / Debug.LogError. BasisDebug routes through Basis's tagged, color-coded logger and respects the project-wide LoggingDisabled toggle so logging can be killed at runtime; bare Debug.Log calls bypass that and will be denied.
• No scene-wide discovery for dependencies — New code is architected so it does not need FindObjectOfType / FindObjectsOfType / GameObject.Find / FindGameObjectsWithTag to locate what it depends on. References are wired in — registered through an existing manager/driver, injected at init, or passed in by the caller — rather than discovered by scanning the scene at runtime. If a scene scan is genuinely unavoidable, justify it under Notes.
• No allocations in hot paths — Per-frame code (Update / LateUpdate / FixedUpdate, simulation loops, jobs, anything called once per frame or more) does not allocate. No new on reference types, no LINQ, no string concatenation/interpolation, no boxing, no foreach over interface-typed collections. Allocate once at init and reuse the buffer.
• No debugging in hot paths — No log calls of any kind on per-frame paths, including BasisDebug. Hot-path logging floods the console and incurs cost on every frame regardless of whether the message is filtered out downstream. If a hot-path log is needed while iterating, gate it behind #if UNITY_EDITOR and remove (or leave gated) before merge.
• Hot-path collection access is optimized — Cache .Count (lists) / .Length (arrays) into a local int before the loop instead of re-reading the property each iteration. Prefer T[] (with a separate length int when the array is over-sized) over List<T> where the data is hot — Unity's mono BCL doesn't expose CollectionsMarshal.AsSpan(List<T>), so a list can't be fed into Span<T> / unsafe paths cleanly. Where the perf justifies it, drop into Span<T> / ref locals / Unsafe.As / unsafe pointer code to skip bounds checks and copies, and call out the invariants you're relying on under Notes so reviewers can sanity-check them.

Testing details

Tick the platforms you actually tested on. Leave the rest unticked — these are informational and do not block merge.

• Windows
• Linux
• Android
• iOS
• macOS

Input / control mode coverage:

• Tested in VR (note headset under Notes)
• Tested in desktop / non-VR mode
• Tested with phone controls (mobile touch input)
• N/A — change does not touch player/XR/input code

Where applicable, confirm these flows still work after your changes:

• Hot-switching (desktop ↔ VR mode swap at runtime)
• Avatar swapping
• Server swapping (joining / leaving / changing servers)
• N/A — change does not touch any of the above

Notes

Validation

Testing was conducted in the editor on Windows 11.
Integrated fMP4 VOD file: https://zipline.space.superneko.net/raw/bbb_sunflower_1080p_30fps_normal_idfmp4.mp4

The test file was encoded using the following options:

ffmpeg -i input.mp4 `
    -map 0:v:0 -map 0:a:0? `
    -c:v libx264 -profile:v high -level:v 4.0 -pix_fmt yuv420p -r 30 `
    -c:a aac -b:a 128k -ar 48000 -ac 2 `
    -video_track_timescale 15360 -movie_timescale 15360 `
    -x264-params "keyint=210:min-keyint=1:scenecut=40:open-gop=0" `
    -movflags +empty_moov+dash+frag_keyframe+default_base_moof+global_sidx+skip_trailer `
    -brand dash `
    -use_editlist 1 `
    out.mp4

• Integrated fMP4 VOD: Pausing and then resuming playback causes a small jump of approximately one second. This matches the behavior of progressive MP4 playback, so there is no regression. Player on Android also works.

• HLS VOD: The video is reported as seekable, but playback becomes stuck after actually seeking. This matches the original behavior, so there is no regression, although it is probably a bug.

• RIST: Validated successfully.

• Split-stream playback: Tested with a video podcast resolved by dlp-native. For some reason, the audio briefly cuts out approximately one second after audio/video synchronization completes. After this interruption, playback continues without further issues.

Limitations

• Seek requests are snapped to the start of the earliest fragment containing that timestamp.
• Currently, this code assumes that a file has exactly one sidx box. at least, I have confirmed that a 1.5-hour video on the website is delivered in this format.
• Seeking to SAP-type-0 (unspecified) fragment may cause visual artifact, but I skipped this check this intentionally. I couldn't find any resource that describes how I encode/demux the test material using ffmpeg.

Native libraries

Base native libraries are built with this patch, based on 59b6e04 librist enabled.

[towneh]

Confirming this one's known — we hit the same on device (Quest, Mux x36xhzz HLS VOD) and had it tracked separately, so agreed: pre-existing, not a regression from this PR.

Light version of what we've seen so far: the segment jump itself works (the VOD seek index rebuilds the fetch queue at the target), but the delivery pace clock doesn't re-anchor on an HLS seek the way a byte-source seek does — so after the jump, playback runs against a stale pacing anchor and stalls. It's on our list; still digging, no fix pinned down yet.

[towneh]

Follow-up: the HLS-TS VOD seek issue mentioned on this PR (forward stall / backward flood after seeking) is now fixed in #956 — the segment-source reposition re-anchors the delivery pace clock at the flushed boundary.

[towneh]

Closing the loop: this is fixed in #956 (merged). One correction to my note above: a re-anchor did fire on HLS seeks, it just lost a race to access units already in flight from before the seek, so the pace anchor could re-base on stale timestamps. #956 moves the flush and re-anchor onto the demux thread at the flushed boundary, the same way the byte-source seek already worked, and both seek directions now behave on the Mux fixture (Windows editor and Quest).