Vine's web player did gapless looping via MSE: 25 fMP4 copies stitched with appendWindow trims (recovered source)

Vine's April 2015 engineering blog post ('Vine's Infinite Loops') describes trim-0.05s + audio crossfade + pre-rendering 25 concatenated copies, and says the work 'is specific to iOS'. The recovered web player source proves the web player ALSO shipped the N-copies technique — via MediaSource Extensions, minus the audio crossfade.

How to verify: load Wayback Machine capture 20150529050307 of any vine.co/v/ permalink (e.g. post OQlWgxzQVBU). The page HTML references two assets on v.cdn.vine.co under /w/f6548e06-assets/: a vendor bundle and the app bundle (vine-ember, Ember app version 2.0.0.2f05f2b3). Fetch the app bundle through Wayback's js_ endpoint, prettify, and read the AMD module 'vine-ember/controllers/playing-vine'.

Mechanics (action mediaSourceLoaded):

• Gate: Chrome > 30 only (mediaSourceEnabledBrowsers: {Chrome: 30}), Opera excluded, and the post must have a format:'dash' variant — a real fragmented MP4 (ftyp dash, mvex/sidx/moof boxes), served from the videos_h264dash/ CDN path.
• Fetch the whole fMP4 as one ArrayBuffer, create ONE SourceBuffer (video/mp4 with avc1.4d401e + mp4a.40.2 codecs), append 25 copies of the entire file.
• Stitch per copy: timestampOffset = bufferedEnd - 0.05, appendWindowStart = bufferedEnd, appendWindowEnd = bufferedEnd + duration - 2*0.05 — i.e. 0.05 s trimmed per end via append windows. The 50 ms head trim conveniently swallows AAC encoder priming (~2112 samples ≈ 48 ms at 44.1 kHz, Apple TN2258), which MSE would otherwise render as junction silence because MSE segment timelines ignore edit lists.
• endOfStream(), then start at currentTime = 0.061 (0.05 trim + 0.011 nudge), play(). Virtual loop boundaries detected by Math.floor(currentTime/duration) increasing on timeupdate. When the ~25 copies run out, the real 'ended' event fires -> setCurrentTime(0); play() — one gapful wrap every 25 loops.
• Exact loop period came from a custom udta metadata box 'com.vineapp.duration' (name 'Vine Duration') embedded in the fMP4, parsed client-side by a tiny mp4 parser in the vendor bundle — because fMP4 moov durations are unreliable.

Other recovered details: fallback loop path was ended -> setCurrentTime(0) -> play() (gapful); mobile used the native loop attribute; a 1 Hz health checker rebooted the player if currentTime froze while playing, plus a post-wrap check (currentTime >= floor(duration) after the seek means it failed -> reboot); a 'loop' was counted when playback passed 0.5 s of each pass (so the first playthrough counted — loops ≈ plays), with anti-bot (two records under 2 s apart disable tracking), cap 25 per post per session, batched POST /api/loops every 30 s, pending counts persisted in localStorage across unload.

Transferable lesson: for seamless A/V looping on the web, a continuous MSE timeline (repeated appends with timestampOffset + appendWindow trims) eliminates the native wrap seek entirely; append windows double as a clean way to drop AAC priming at junctions without re-encoding.