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Zoom jumping (device: min 0.5 / max 2.47 while zooming near 1): the per-frame scale clamp limited single-frame spikes but not multi-frame runs. Root cause is pointer-count transitions — on Windows touch the two fingers land/lift at different times and digitizers drop/re-acquire touches, and stock InteractiveViewer keeps _scaleStart/_referenceFocalPoint from the OLD finger set, so the next frame jumps. PenInteractiveViewer now re-baselines (and skips the transitional frame) whenever details.pointerCount changes. The per-frame clamp stays as a secondary guard. Buttons (device evidence: btn=1 for tip-down, side-button, AND inverted; kind never becomes invertedStylus): Flutter does NOT surface the barrel/eraser/inverted state at all — unreachable from Dart. The only path is the native badnote/pen plugin, which was SILENT because WM_POINTER is delivered to the Flutter CHILD view window, not the top-level FlutterWindow where ObservePenMessage was hooked. Fix: subclass the child HWND (SetWindowSubclass + comctl32) and observe its WM_POINTER messages, passing every message through unchanged via DefSubclassProc (observation-only, input behavior preserved). This is what should finally feed GetPointerPenInfo penFlags + tilt to the channel — to be confirmed on-device with the diagnostic (btn / kind / tilt readout). Dart: analyze clean, 66/66 tests, linux build green. Native compiles on Windows CI. Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>