feat(pen): forked InteractiveViewer (stylus-exclusive draw) + zoom diagnostic

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   prior _lastStylus hover hack is removed (superseded).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>

lib/editor/canvas/pen_canvas.dart

@@ -26,6 +26,7 @@ import '../engine/stroke_geometry.dart' show kDefaultPenThinning;
 import '../input/pen_config.dart';
 import '../input/pen_input_service.dart';
 import 'ink_painters.dart';
+import 'pen_interactive_viewer.dart';
 import 'pen_stroke.dart';
 
 /// The active tool on the pen canvas.
@@ -132,13 +133,6 @@ class _PenCanvasState extends State<PenCanvas> {
   /// mode / the pen is not near the page. Drives [EraserPreviewPainter].
   PenPoint? _eraserCursor;
 
-  /// True when the most recent pointer was a stylus. Set on stylus HOVER, which
-  /// precedes contact on Windows, so the InteractiveViewer's pan is already
-  /// disabled BEFORE the stroke starts — killing the 1-frame pan-steal that
-  /// corrupts fast strokes (the "写字识别成单击" feel bug). A finger/mouse down
-  /// flips it back so finger-pan still works.
-  bool _lastStylus = false;
-
   /// True when the eraser would act (eraser tool selected, or a barrel/inverted
   /// eraser signal is live).
   bool get _isEraserMode =>
@@ -153,11 +147,6 @@ class _PenCanvasState extends State<PenCanvas> {
       ? 1.0
       : widget.pageSize.height / widget.pageSize.width;
 
-  void _setLastStylus(bool v) {
-    if (_lastStylus == v) return;
-    setState(() => _lastStylus = v);
-  }
-
   /// Update (or clear) the eraser-preview cursor from a global pointer position.
   void _updateEraserCursor(Offset globalPosition) {
     if (_isEraserMode) {
@@ -416,7 +405,6 @@ class _PenCanvasState extends State<PenCanvas> {
 
   void _onPointerHover(PointerHoverEvent event) {
     if (_isStylus(event.kind)) {
-      _setLastStylus(true);
       _emitPenDebug(event);
       // Fire edge-triggered button actions (undo / toggleTool) on hover so a
       // mapped barrel press works without first touching down.
@@ -430,7 +418,6 @@ class _PenCanvasState extends State<PenCanvas> {
 
   void _onPointerDown(PointerDownEvent event) {
     if (event.kind == PointerDeviceKind.trackpad) return;
-    _setLastStylus(_isStylus(event.kind));
     if (_isStylus(event.kind)) {
       _emitPenDebug(event);
       // Fire edge-triggered button actions for a direct pen-down (no prior
@@ -482,17 +469,12 @@ class _PenCanvasState extends State<PenCanvas> {
 
   @override
   Widget build(BuildContext context) {
-    // Pan rules (pen-first):
-    //  - A 2+ pointer pinch ALWAYS pans (the focal-point translation is part of
-    //    zooming), regardless of pen state.
-    //  - Otherwise pan only when NOT mid-stroke AND the last pointer was not a
-    //    stylus. Because Windows fires stylus HOVER before contact, _lastStylus
-    //    is already true when the pen touches down, so the InteractiveViewer's
-    //    pan is disabled BEFORE the stroke's first move — no 1-frame pan-steal
-    //    that would corrupt a fast flick into a tap.
-    final panEnabled = _activePointers.length >= 2
-        ? true
-        : (_drawPointer == null && !_lastStylus);
+    // The PEN never reaches PenInteractiveViewer's recognizer (it excludes
+    // stylus), so a stylus stroke can never be stolen as a pan. panEnabled only
+    // governs touch/mouse: suppress pan while a single-finger / mouse stroke is
+    // in progress (finger-drawing mode); a 2nd pointer cancels the stroke first
+    // so a pinch re-enables pan/zoom immediately.
+    final panEnabled = _drawPointer == null;
 
     return Listener(
       onPointerHover: _onPointerHover,
@@ -500,14 +482,12 @@ class _PenCanvasState extends State<PenCanvas> {
       onPointerMove: _onPointerMove,
       onPointerUp: _onPointerUp,
       onPointerCancel: _onPointerCancel,
-      child: InteractiveViewer(
+      child: PenInteractiveViewer(
         transformationController: widget.transformationController,
         minScale: widget.minScale,
         maxScale: widget.maxScale,
         panEnabled: panEnabled,
         scaleEnabled: true,
-        constrained: false,
-        boundaryMargin: const EdgeInsets.all(double.infinity),
         child: SizedBox(
           width: widget.pageSize.width,
           height: widget.pageSize.height,

lib/editor/canvas/pen_editor_screen.dart

@@ -95,6 +95,13 @@ class _PenEditorScreenState extends State<PenEditorScreen> {
   String _penDebug = '';
   bool _showPenDebug = false;
 
+  /// Live zoom-scale diagnostic (only tracked while the overlay is on), so we
+  /// can capture the value the pinch flash jumps to. `_zoomMin/_zoomMax` record
+  /// the extremes seen since the overlay was last enabled.
+  double _zoomNow = 1.0;
+  double _zoomMin = double.infinity;
+  double _zoomMax = 0.0;
+
   // Tool state.
   CanvasTool _tool = CanvasTool.pen;
   Color _color = Colors.black;
@@ -119,11 +126,23 @@ class _PenEditorScreenState extends State<PenEditorScreen> {
     // Begin listening to the native Windows pen plugin (barrel/eraser/tilt).
     // No-op on platforms without the plugin (W3).
     PenInputService.instance.start();
+    _transform.addListener(_onTransformDebug);
     _initPersistence();
     _initPenConfig();
     _open();
   }
 
+  /// Track the live zoom scale for the diagnostic overlay (no-op when off).
+  void _onTransformDebug() {
+    if (!_showPenDebug) return;
+    final s = _transform.value.getMaxScaleOnAxis();
+    setState(() {
+      _zoomNow = s;
+      if (s < _zoomMin) _zoomMin = s;
+      if (s > _zoomMax) _zoomMax = s;
+    });
+  }
+
   Future<void> _initPenConfig() async {
     final controller = await PenConfigController.load();
     if (!mounted) {
@@ -221,6 +240,7 @@ class _PenEditorScreenState extends State<PenEditorScreen> {
       scheduler.dispose();
     }
     _document?.dispose();
+    _transform.removeListener(_onTransformDebug);
     _transform.dispose();
     _penConfig?.dispose();
     PenInputService.instance.stop();
@@ -437,9 +457,10 @@ class _PenEditorScreenState extends State<PenEditorScreen> {
                       padding: const EdgeInsets.symmetric(
                           horizontal: 10, vertical: 6),
                       child: Text(
-                        _penDebug.isEmpty
-                            ? 'hover / draw with the pen…'
-                            : _penDebug,
+                        '${_penDebug.isEmpty ? 'hover / draw with the pen…' : _penDebug}'
+                        '\nzoom=${_zoomNow.toStringAsFixed(2)}  '
+                        'min=${_zoomMin.isFinite ? _zoomMin.toStringAsFixed(2) : '-'}  '
+                        'max=${_zoomMax.toStringAsFixed(2)}',
                         style: TextStyle(
                           fontFamily: 'monospace',
                           fontSize: 12,
@@ -600,8 +621,15 @@ class _PenEditorScreenState extends State<PenEditorScreen> {
             _ToolButton(
               icon: Icons.bug_report_outlined,
               selected: _showPenDebug,
-              tooltip: 'Pen pressure diagnostic',
-              onPressed: () => setState(() => _showPenDebug = !_showPenDebug),
+              tooltip: 'Pen + zoom diagnostic',
+              onPressed: () => setState(() {
+                _showPenDebug = !_showPenDebug;
+                if (_showPenDebug) {
+                  _zoomMin = double.infinity;
+                  _zoomMax = 0.0;
+                  _zoomNow = _transform.value.getMaxScaleOnAxis();
+                }
+              }),
             ),
           ],
         ),

lib/editor/canvas/pen_interactive_viewer.dart

@@ -0,0 +1,411 @@
+// lib/editor/canvas/pen_interactive_viewer.dart
+//
+// A focused fork of Flutter 3.44's InteractiveViewer, adapted for the pen-first
+// canvas (clean-room model shared with Saber). Two deliberate changes vs stock:
+//
+//  1. The pan/zoom ScaleGestureRecognizer is restricted to NON-stylus devices
+//     (`supportedDevices` excludes stylus / invertedStylus). The pen therefore
+//     never reaches this recognizer — it only draws via the canvas `Listener`.
+//     This removes the gesture-arena fight and, crucially, the one-frame
+//     "pan-steal" where a stylus stroke's first frame was consumed as a pan
+//     (the "写字识别成单击" feel bug) because stock InteractiveViewer's
+//     `panEnabled` only updated a frame after the stroke had begun.
+//
+//  2. The per-frame scale change is clamped (`_kMin/_MaxScaleChangePerFrame`).
+//     Stock InteractiveViewer already damps focal jitter and guards the pan
+//     branch, but a single-frame multi-touch glitch can still spike
+//     `details.scale`, popping the zoom bigger/smaller for one frame and then
+//     snapping back (the reported pinch flicker). Clamping the per-update change
+//     swallows that spike without affecting a real (gradual) pinch, since scale
+//     is tracked absolutely from gesture start and simply catches up next frame.
+//
+// Everything else (scale-about-focal math, pan, fling inertia, mouse-wheel zoom)
+// is Flutter's proven logic. The boundary/rotation/panAxis machinery is dropped
+// because this canvas always uses an infinite boundary, free pan, and no
+// rotation — so that code was provably a no-op here.
+
+import 'dart:math' as math;
+
+import 'package:flutter/foundation.dart' show clampDouble;
+import 'package:flutter/gestures.dart';
+import 'package:flutter/physics.dart';
+import 'package:flutter/widgets.dart';
+
+/// Devices allowed to pan/zoom. Stylus + invertedStylus are excluded so the pen
+/// is owned exclusively by the drawing `Listener`.
+const Set<PointerDeviceKind> _kPanZoomDevices = <PointerDeviceKind>{
+  PointerDeviceKind.touch,
+  PointerDeviceKind.mouse,
+  PointerDeviceKind.trackpad,
+  PointerDeviceKind.unknown,
+};
+
+/// Per-frame multiplicative scale-change clamp (flicker guard).
+const double _kMaxScaleChangePerFrame = 1.35;
+const double _kMinScaleChangePerFrame = 1 / _kMaxScaleChangePerFrame;
+
+const double _kDrag = 0.0000135;
+
+enum _GestureType { pan, scale }
+
+/// Pan + zoom for the pen canvas. The pen never reaches this widget's gesture
+/// recognizer; only touch / mouse / trackpad pan and zoom the shared transform.
+class PenInteractiveViewer extends StatefulWidget {
+  const PenInteractiveViewer({
+    super.key,
+    required this.transformationController,
+    required this.child,
+    this.minScale = 0.5,
+    this.maxScale = 8.0,
+    this.panEnabled = true,
+    this.scaleEnabled = true,
+    this.scaleFactor = kDefaultMouseScrollToScaleFactor,
+    this.interactionEndFrictionCoefficient = _kDrag,
+  })  : assert(minScale > 0),
+        assert(maxScale >= minScale);
+
+  final TransformationController transformationController;
+  final Widget child;
+  final double minScale;
+  final double maxScale;
+  final bool panEnabled;
+  final bool scaleEnabled;
+  final double scaleFactor;
+  final double interactionEndFrictionCoefficient;
+
+  @override
+  State<PenInteractiveViewer> createState() => _PenInteractiveViewerState();
+}
+
+class _PenInteractiveViewerState extends State<PenInteractiveViewer>
+    with TickerProviderStateMixin {
+  TransformationController get _transformer => widget.transformationController;
+
+  final GlobalKey _childKey = GlobalKey();
+  Animation<Offset>? _animation;
+  Animation<double>? _scaleAnimation;
+  late Offset _scaleAnimationFocalPoint;
+  late AnimationController _controller;
+  late AnimationController _scaleController;
+  Offset? _referenceFocalPoint;
+  double? _scaleStart;
+  _GestureType? _gestureType;
+
+  // --- Matrix helpers (infinite boundary → no clamping to bounds) -----------
+
+  Matrix4 _matrixTranslate(Matrix4 matrix, Offset translation) {
+    if (translation == Offset.zero) return matrix.clone();
+    return matrix.clone()
+      ..translateByDouble(translation.dx, translation.dy, 0, 1);
+  }
+
+  Matrix4 _matrixScale(Matrix4 matrix, double scale) {
+    if (scale == 1.0) return matrix.clone();
+    assert(scale != 0.0);
+    final double currentScale = _transformer.value.getMaxScaleOnAxis();
+    final double clampedTotalScale = clampDouble(
+      currentScale * scale,
+      widget.minScale,
+      widget.maxScale,
+    );
+    final double clampedScale = clampedTotalScale / currentScale;
+    return matrix.clone()
+      ..scaleByDouble(clampedScale, clampedScale, clampedScale, 1);
+  }
+
+  bool _gestureIsSupported(_GestureType? gestureType) => switch (gestureType) {
+        _GestureType.scale => widget.scaleEnabled,
+        _GestureType.pan || null => widget.panEnabled,
+      };
+
+  _GestureType _getGestureType(ScaleUpdateDetails details) {
+    final double scale = widget.scaleEnabled ? details.scale : 1.0;
+    return (scale - 1).abs() > 0 ? _GestureType.scale : _GestureType.pan;
+  }
+
+  // --- Gesture lifecycle ----------------------------------------------------
+
+  void _onScaleStart(ScaleStartDetails details) {
+    if (_controller.isAnimating) {
+      _controller.stop();
+      _controller.reset();
+      _animation?.removeListener(_handleInertiaAnimation);
+      _animation = null;
+    }
+    if (_scaleController.isAnimating) {
+      _scaleController.stop();
+      _scaleController.reset();
+      _scaleAnimation?.removeListener(_handleScaleAnimation);
+      _scaleAnimation = null;
+    }
+    _gestureType = null;
+    _scaleStart = _transformer.value.getMaxScaleOnAxis();
+    _referenceFocalPoint = _transformer.toScene(details.localFocalPoint);
+  }
+
+  void _onScaleUpdate(ScaleUpdateDetails details) {
+    final double scale = _transformer.value.getMaxScaleOnAxis();
+    _scaleAnimationFocalPoint = details.localFocalPoint;
+    final Offset focalPointScene = _transformer.toScene(details.localFocalPoint);
+
+    if (_gestureType == _GestureType.pan) {
+      // A 2-finger gesture can start with no scale change; allow re-typing it.
+      _gestureType = _getGestureType(details);
+    } else {
+      _gestureType ??= _getGestureType(details);
+    }
+    if (!_gestureIsSupported(_gestureType)) return;
+
+    switch (_gestureType!) {
+      case _GestureType.scale:
+        assert(_scaleStart != null);
+        final double desiredScale = _scaleStart! * details.scale;
+        // Flicker guard: clamp the per-frame change so a single-frame touch
+        // jitter can't pop the zoom and snap back. Absolute tracking means a
+        // real pinch just resumes next frame.
+        final double scaleChange = clampDouble(
+          desiredScale / scale,
+          _kMinScaleChangePerFrame,
+          _kMaxScaleChangePerFrame,
+        );
+        _transformer.value = _matrixScale(_transformer.value, scaleChange);
+
+        // Keep the focal point anchored under the fingers across the scale.
+        final Offset focalPointSceneScaled =
+            _transformer.toScene(details.localFocalPoint);
+        _transformer.value = _matrixTranslate(
+          _transformer.value,
+          focalPointSceneScaled - _referenceFocalPoint!,
+        );
+
+        // Re-anchor only when the rounded focal actually drifted (jitter damp).
+        final Offset focalPointSceneCheck =
+            _transformer.toScene(details.localFocalPoint);
+        if (_round(_referenceFocalPoint!) != _round(focalPointSceneCheck)) {
+          _referenceFocalPoint = focalPointSceneCheck;
+        }
+
+      case _GestureType.pan:
+        assert(_referenceFocalPoint != null);
+        // Throw away near-scale frames so a stale reference can't jump the pan.
+        if (details.scale != 1.0) return;
+        final Offset translationChange =
+            focalPointScene - _referenceFocalPoint!;
+        _transformer.value =
+            _matrixTranslate(_transformer.value, translationChange);
+        _referenceFocalPoint = _transformer.toScene(details.localFocalPoint);
+    }
+  }
+
+  void _onScaleEnd(ScaleEndDetails details) {
+    _scaleStart = null;
+    _referenceFocalPoint = null;
+    _animation?.removeListener(_handleInertiaAnimation);
+    _scaleAnimation?.removeListener(_handleScaleAnimation);
+    _controller.reset();
+    _scaleController.reset();
+
+    if (!_gestureIsSupported(_gestureType)) return;
+
+    switch (_gestureType) {
+      case _GestureType.pan:
+        if (details.velocity.pixelsPerSecond.distance < kMinFlingVelocity) {
+          return;
+        }
+        final translationVector = _transformer.value.getTranslation();
+        final Offset translation =
+            Offset(translationVector.x, translationVector.y);
+        final FrictionSimulation frictionSimulationX = FrictionSimulation(
+          widget.interactionEndFrictionCoefficient,
+          translation.dx,
+          details.velocity.pixelsPerSecond.dx,
+        );
+        final FrictionSimulation frictionSimulationY = FrictionSimulation(
+          widget.interactionEndFrictionCoefficient,
+          translation.dy,
+          details.velocity.pixelsPerSecond.dy,
+        );
+        final double tFinal = _getFinalTime(
+          details.velocity.pixelsPerSecond.distance,
+          widget.interactionEndFrictionCoefficient,
+        );
+        _animation = Tween<Offset>(
+          begin: translation,
+          end: Offset(frictionSimulationX.finalX, frictionSimulationY.finalX),
+        ).animate(CurvedAnimation(parent: _controller, curve: Curves.decelerate));
+        _controller.duration = Duration(milliseconds: (tFinal * 1000).round());
+        _animation!.addListener(_handleInertiaAnimation);
+        _controller.forward();
+      case _GestureType.scale:
+        if (details.scaleVelocity.abs() < 0.1) return;
+        final double scale = _transformer.value.getMaxScaleOnAxis();
+        final FrictionSimulation frictionSimulation = FrictionSimulation(
+          widget.interactionEndFrictionCoefficient * widget.scaleFactor,
+          scale,
+          details.scaleVelocity / 10,
+        );
+        final double tFinal = _getFinalTime(
+          details.scaleVelocity.abs(),
+          widget.interactionEndFrictionCoefficient,
+          effectivelyMotionless: 0.1,
+        );
+        _scaleAnimation = Tween<double>(
+          begin: scale,
+          end: frictionSimulation.x(tFinal),
+        ).animate(
+            CurvedAnimation(parent: _scaleController, curve: Curves.decelerate));
+        _scaleController.duration = Duration(milliseconds: (tFinal * 1000).round());
+        _scaleAnimation!.addListener(_handleScaleAnimation);
+        _scaleController.forward();
+      case null:
+        break;
+    }
+  }
+
+  // --- Mouse wheel / trackpad zoom ------------------------------------------
+
+  void _receivedPointerSignal(PointerSignalEvent event) {
+    final double scaleChange;
+    if (event is PointerScrollEvent) {
+      if (event.kind == PointerDeviceKind.trackpad) {
+        // Trackpad scroll → pan.
+        if (!_gestureIsSupported(_GestureType.pan)) return;
+        final Offset localDelta = PointerEvent.transformDeltaViaPositions(
+          untransformedEndPosition: event.position + event.scrollDelta,
+          untransformedDelta: event.scrollDelta,
+          transform: event.transform,
+        );
+        final Offset focalPointScene = _transformer.toScene(event.localPosition);
+        final Offset newFocalPointScene =
+            _transformer.toScene(event.localPosition - localDelta);
+        _transformer.value = _matrixTranslate(
+          _transformer.value,
+          newFocalPointScene - focalPointScene,
+        );
+        return;
+      }
+      if (event.scrollDelta.dy == 0.0) return;
+      scaleChange = math.exp(-event.scrollDelta.dy / widget.scaleFactor);
+    } else if (event is PointerScaleEvent) {
+      scaleChange = event.scale;
+    } else {
+      return;
+    }
+    if (!_gestureIsSupported(_GestureType.scale)) return;
+
+    final Offset focalPointScene = _transformer.toScene(event.localPosition);
+    _transformer.value = _matrixScale(_transformer.value, scaleChange);
+    final Offset focalPointSceneScaled =
+        _transformer.toScene(event.localPosition);
+    _transformer.value = _matrixTranslate(
+      _transformer.value,
+      focalPointSceneScaled - focalPointScene,
+    );
+  }
+
+  void _handleInertiaAnimation() {
+    if (!_controller.isAnimating) {
+      _animation?.removeListener(_handleInertiaAnimation);
+      _animation = null;
+      _controller.reset();
+      return;
+    }
+    final translationVector = _transformer.value.getTranslation();
+    final Offset translation = Offset(translationVector.x, translationVector.y);
+    _transformer.value = _matrixTranslate(
+      _transformer.value,
+      _transformer.toScene(_animation!.value) - _transformer.toScene(translation),
+    );
+  }
+
+  void _handleScaleAnimation() {
+    if (!_scaleController.isAnimating) {
+      _scaleAnimation?.removeListener(_handleScaleAnimation);
+      _scaleAnimation = null;
+      _scaleController.reset();
+      return;
+    }
+    final double desiredScale = _scaleAnimation!.value;
+    final double scaleChange =
+        desiredScale / _transformer.value.getMaxScaleOnAxis();
+    final Offset referenceFocalPoint =
+        _transformer.toScene(_scaleAnimationFocalPoint);
+    _transformer.value = _matrixScale(_transformer.value, scaleChange);
+    final Offset focalPointSceneScaled =
+        _transformer.toScene(_scaleAnimationFocalPoint);
+    _transformer.value = _matrixTranslate(
+      _transformer.value,
+      focalPointSceneScaled - referenceFocalPoint,
+    );
+  }
+
+  void _handleTransformation() => setState(() {});
+
+  @override
+  void initState() {
+    super.initState();
+    _controller = AnimationController(vsync: this);
+    _scaleController = AnimationController(vsync: this);
+    _transformer.addListener(_handleTransformation);
+  }
+
+  @override
+  void didUpdateWidget(PenInteractiveViewer oldWidget) {
+    super.didUpdateWidget(oldWidget);
+    if (oldWidget.transformationController != widget.transformationController) {
+      oldWidget.transformationController.removeListener(_handleTransformation);
+      widget.transformationController.addListener(_handleTransformation);
+    }
+  }
+
+  @override
+  void dispose() {
+    _controller.dispose();
+    _scaleController.dispose();
+    _transformer.removeListener(_handleTransformation);
+    super.dispose();
+  }
+
+  @override
+  Widget build(BuildContext context) {
+    Widget child = Transform(
+      transform: _transformer.value,
+      child: KeyedSubtree(key: _childKey, child: widget.child),
+    );
+    child = OverflowBox(
+      alignment: Alignment.topLeft,
+      minWidth: 0.0,
+      minHeight: 0.0,
+      maxWidth: double.infinity,
+      maxHeight: double.infinity,
+      child: child,
+    );
+    child = ClipRect(child: child);
+
+    return Listener(
+      onPointerSignal: _receivedPointerSignal,
+      child: GestureDetector(
+        behavior: HitTestBehavior.opaque,
+        supportedDevices: _kPanZoomDevices,
+        onScaleStart: _onScaleStart,
+        onScaleUpdate: _onScaleUpdate,
+        onScaleEnd: _onScaleEnd,
+        trackpadScrollCausesScale: false,
+        trackpadScrollToScaleFactor: Offset(0, -1 / widget.scaleFactor),
+        child: child,
+      ),
+    );
+  }
+}
+
+double _getFinalTime(double velocity, double drag,
+    {double effectivelyMotionless = 10}) {
+  return math.log(effectivelyMotionless / velocity) / math.log(drag / 100);
+}
+
+Offset _round(Offset offset) {
+  return Offset(
+    double.parse(offset.dx.toStringAsFixed(9)),
+    double.parse(offset.dy.toStringAsFixed(9)),
+  );
+}