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Root causes fixed: - Sequencer: replaced setTimeout note-off with Tone.Transport.scheduleOnce for sample-accurate timing instead of main-thread-dependent setTimeout - Sequencer + PianoRoll: decoupled visual updates from audio callbacks. Audio clock only writes to refs, RAF loop reads refs for visual step indicator. No more React setState inside Tone.Clock callbacks. - audioEngine: added connection lookup cache (Map) to replace O(n²) array iterations in setSequencerSignals/triggerKeyboard. Cache rebuilds lazily only when connections change. These changes eliminate the feedback loop where: audio callback → setState → React render → main thread blocks → setTimeout delayed → note-off late → drift compounds Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>