Proposal: bandwidth-aware "Auto" quality (today Auto is simply the highest level) and a rate-scaled MSE forward buffer #214

Severity: medium

Where: chrome/player/players/LevelManager.mjs (getDesiredVideoHeight: if (defaultQuality === 'Auto') return Infinity;, documented as deliberate: "Always max out rather than scaling to the screen"), chrome/player/players/hls/HLSPlayer.mjs (abandon rules disabled, preserveManualLevelOnError: true, maxBufferLength: 10), chrome/player/players/dash/DashPlayer.mjs (autoSwitchBitrate: {audio: false, video: false}), chrome/player/players/mp4/MP4Player.mjs (maxBufferLength)

Today

Both libraries' ABR is switched off and the level is pinned as manual, and Auto picks the top rendition. A connection slower than the top rendition rebuffers every few seconds forever (the 300 s pre-buffer only helps when average throughput exceeds the bitrate); a 4K rendition is fetched for a 1080p window (hls.js's capLevelToPlayerSize is inert for manual levels). DownloadManager already measures throughput (getSpeed(), the speed test with up to 6 downloaders).

Proposal

Keep the pinned-level design and only automate the pick: when defaultQuality === 'Auto', choose the highest level whose bitrate <= about 0.8 x measured throughput once the speed test has settled, re-evaluated in mainloop with hysteresis (step up only after N seconds above the next level's bitrate; step down when buffered-ahead falls under a threshold), optionally capped by screen.height x devicePixelRatio; explicit user choices stay manual.

Second, the hls.js forward buffer is fixed at 10 s regardless of rate: at 4-8x that is 1.25-2.5 s of wall time, so any hiccup in the transmux worker or in reading a fragment back from OPFS shows as a stall although 300 s are on disk. Scale hls.config.maxBufferLength/maxMaxBufferLength (and MP4Player's maxBufferLength) with the playback rate in the playbackRate setter, e.g. 10 x rate capped at 60 s; the back buffer can stay at 10 s.

Confidence: high that ABR is absent; medium on the policy and on the buffer claim (not measured).

From a line-by-line code review of the tree at 41363fd (2026-10-02). Each claim above was checked against the source before filing; the reproduction steps and line references are as of that commit.

Generated by Claude Code