[markm39]

Problem

After 0.3.8 (continuous documents), on iPad:

• Writing lags during autosave. Every save serialized the whole document on the UI thread (iOS main / Android GL), base64- and JSON-encoded it there, sent ~20 MB through JS, then decoded it again to split it into pages. Strokes drawn during a save were delayed.
• Pan/zoom lags (both apps). Continuous mode sent a new crop rect every frame, so native re-rendered (and resized) the ink drawable ~60 times a second.

Fix

• snapshotDrawing() copies committed strokes under the engine lock; serialization runs on a background thread (iOS getBase64Data, Android getBase64Data/persistEngineToFile).
• New native captureDocument(viewTag, pageCount, pageHeight) (iOS + Android) partitions the stroke copy off-thread; ContinuousDocumentCanvas uses it. No whole-document round trip through JS.
• resolveDocumentRenderTarget: overscanned crop reused until the view leaves it, resolution held during a gesture, nearest crisp bucket on settle, never above the native 4096 px cap. ContinuousDocumentCanvas no longer re-renders its subtree on each viewport frame.
• Typed DocumentChangedDuringCaptureError / isDocumentCaptureSuperseded for superseded saves.

Evidence (iPad Pro 13" M5 simulator, Release, 100-page × 200-stroke fixture — not physical-iPad numbers)

	before (0.3.8)	after
Main-thread stall per autosave (median / max, 20 saves)	59 / 63 ms	6 / 9 ms
Main-thread save time over 35 s of writing	1,148 ms	114 ms
Background save CPU over 35 s of writing	7.46 s	2.36 s
OpenNotes scroll+zoom test, app CPU	3.47 s	1.68 s
Writing test peak memory	720 MB	644 MB

Mac engine bench, 20k strokes: serialize 23.8 ms (old main-thread work) vs snapshot copy 1.5 ms.

Tests

• jest 156 passed, tsc clean; new documentRenderTarget tests, capture args test.
• Native smokes: serialization, eraser, selection, zoom, document-jni, new test:native:snapshot-smoke (snapshot encodes identically to live serialization; unaffected by strokes drawn while encoding off-thread). Added to CI.
• Android: Kotlin compiled against RN 0.81 classes (no new errors vs main); JNI compiled; no device build available locally.
• OpenNotes Release UI tests on simulator: cross-page stroke undo/redo/save/relaunch/export, cross-page text and asset history, rotated image crop, dense interaction audit, invalid-payload recovery — all pass.