Replace stock InteractiveViewer with PenInteractiveViewer, a focused fork of
Flutter 3.44's InteractiveViewer for our config (constrained=false, infinite
boundary, no rotation — that machinery dropped as a no-op here). Two deliberate
changes, grounded in the Rnote/Saber research:
1. The pan/zoom ScaleGestureRecognizer excludes stylus/invertedStylus via
`supportedDevices`. The pen never reaches it, so a stylus stroke can no longer
be stolen as a pan on its first frame (the "写字识别成单击" feel bug, caused by
stock IV's panEnabled updating a frame after the stroke began). Drawing is
owned solely by the canvas Listener; no arena fight, no panEnabled lag. The
prior _lastStylus hover hack is removed (superseded).
2. Per-frame scale change is clamped (×0.74..×1.35). Stock IV already damps focal
jitter and guards the pan branch, but a single-frame multi-touch glitch could
still spike details.scale, popping the zoom bigger/smaller and snapping back
(the reported pinch flicker). Clamping swallows the spike; a real (gradual)
pinch is unaffected since scale tracks absolutely from gesture start. Cap is
far above any real pinch (~1.1-1.2x/frame), so no felt lag.
Everything else (scale-about-focal, pan, fling inertia, mouse-wheel zoom) is
Flutter's proven logic verbatim.
Also add an on-device input diagnostic (bug-report toggle): the existing pen
readout already prints kind/pressure/buttons; now it also shows live
zoom=now/min/max so the next device test captures (a) whether the side/eraser
button arrives as buttons/invertedStylus, and (b) the value any residual pinch
flash jumps to. 66/66 tests, analyze clean, linux build green.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
