fix(zoom): kill re-baseline pinch pop

Device log showed a single-frame scale pop (cur 0.504->0.694, a
+38% jump UP while the pinch was still shrinking).

Root cause: the absolute mapping targetScale = scaleStart *
details.scale is only valid when details.scale is 1.0 at the
moment scaleStart is captured. That holds at gesture start, but
on a mid-gesture re-baseline (a finger blips 2->1->2, routine on
Windows touch) a fresh scaleStart got multiplied by the
recognizer's still-cumulative details.scale, popping the zoom
then snapping back.

Fix: track rawScaleAtBaseline and normalize details.scale against
it so the cumulative reads 1.0 at every baseline. Extracted
absolutePinchScale() pure solver + 5 unit tests covering the
exact re-baseline scenario.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

lib/editor/canvas/pen_interactive_viewer.dart

@@ -32,6 +32,7 @@ import 'package:flutter/physics.dart';
 import 'package:flutter/widgets.dart';
 
 import 'input_diagnostics.dart';
+import 'pinch_scale_solver.dart';
 
 /// Devices allowed to pan/zoom. Stylus + invertedStylus are excluded so the pen
 /// is owned exclusively by the drawing `Listener`.
@@ -116,6 +117,18 @@ class _PenInteractiveViewerState extends State<PenInteractiveViewer>
   double _lastRawScale = 1.0;
   double _lastAppliedScale = 1.0;
 
+  /// The recognizer's cumulative `details.scale` AT THE CURRENT BASELINE (the
+  /// gesture start, or the last pointer-count re-baseline). The absolute target
+  /// is `_scaleStart * (details.scale / _rawScaleAtBaseline)`: dividing by this
+  /// re-normalizes the cumulative scale so it reads 1.0 at the baseline moment.
+  ///
+  /// Without this, a mid-gesture re-baseline (a finger blips 2→1→2 — routine on
+  /// Windows touch) captured a fresh `_scaleStart` but left `details.scale` at
+  /// its un-normalized cumulative value, so the next frame computed
+  /// `_scaleStart * 0.40` and the zoom popped to a wrong scale then snapped back
+  /// (the reported flicker). Normalizing kills that pop at the source.
+  double _rawScaleAtBaseline = 1.0;
+
   // --- Matrix helpers (infinite boundary → no clamping to bounds) -----------
 
   Matrix4 _matrixTranslate(Matrix4 matrix, Offset translation) {
@@ -169,6 +182,7 @@ class _PenInteractiveViewerState extends State<PenInteractiveViewer>
     _referenceFocalPoint = _transformer.toScene(details.localFocalPoint);
     _lastRawScale = 1.0;
     _lastAppliedScale = _scaleStart!;
+    _rawScaleAtBaseline = 1.0;
   }
 
   void _onScaleUpdate(ScaleUpdateDetails details) {
@@ -184,6 +198,10 @@ class _PenInteractiveViewerState extends State<PenInteractiveViewer>
       _referenceFocalPoint = _transformer.toScene(details.localFocalPoint);
       _lastRawScale = details.scale;
       _lastAppliedScale = _scaleStart!;
+      // Re-anchor the absolute mapping: from here, cumulative scale is measured
+      // relative to THIS frame's details.scale (so the next good frame starts
+      // from _scaleStart, not _scaleStart * a stale cumulative value).
+      _rawScaleAtBaseline = details.scale;
       InputDiagnostics.instance.recordRebaseline();
       return;
     }
@@ -242,10 +260,15 @@ class _PenInteractiveViewerState extends State<PenInteractiveViewer>
         // for a pure scale+translate matrix — no inversion, no live read-back —
         // so an interleaved/transient matrix write can't survive into the next
         // frame: every frame is fully re-derived from clean inputs.
-        final double targetScale = clampDouble(
-          _scaleStart! * details.scale,
-          widget.minScale,
-          widget.maxScale,
+        // Absolute target scale, normalized against the baseline so a
+        // mid-gesture re-baseline (finger blip) can't pop the zoom. See
+        // pinch_scale_solver.dart for the full rationale.
+        final double targetScale = absolutePinchScale(
+          scaleStart: _scaleStart!,
+          rawScaleAtBaseline: _rawScaleAtBaseline,
+          rawScale: details.scale,
+          minScale: widget.minScale,
+          maxScale: widget.maxScale,
         );
         final Offset focal = details.localFocalPoint;
         final double tx = focal.dx - targetScale * _referenceFocalPoint!.dx;

lib/editor/canvas/pinch_scale_solver.dart

@@ -0,0 +1,41 @@
+// lib/editor/canvas/pinch_scale_solver.dart
+//
+// Pure math for the pen canvas's absolute pinch-zoom. Extracted so the
+// re-baseline behavior (the subtle part) can be unit-tested without simulating
+// a flaky multi-pointer gesture.
+//
+// The pinch is driven ABSOLUTELY: the scale shown is always
+//   scaleStart * (rawScale / rawScaleAtBaseline)
+// where `scaleStart` is the matrix scale captured at the current baseline and
+// `rawScaleAtBaseline` is the recognizer's cumulative `details.scale` at that
+// same baseline. Dividing by `rawScaleAtBaseline` re-normalizes the cumulative
+// scale so it reads 1.0 at the baseline instant.
+//
+// Why this matters: a baseline is captured at gesture start AND on every
+// pointer-count change (a finger blips 2→1→2, routine on Windows touch). At
+// gesture start `details.scale` is 1.0, so a naive `scaleStart * rawScale` is
+// correct. But at a MID-GESTURE re-baseline `details.scale` is whatever the
+// pinch has accumulated (e.g. 0.40) — multiplying the fresh `scaleStart` by
+// that stale 0.40 popped the zoom to a wrong scale and snapped back (the
+// reported flicker). Normalizing against `rawScaleAtBaseline` removes the pop.
+
+import 'package:flutter/foundation.dart' show clampDouble;
+
+/// Returns the absolute target scale for a pinch frame.
+///
+/// [scaleStart] — matrix scale captured at the current baseline.
+/// [rawScaleAtBaseline] — recognizer cumulative `details.scale` at that
+///   baseline (1.0 at gesture start; the live value at a re-baseline).
+/// [rawScale] — the recognizer's current cumulative `details.scale`.
+/// Result is clamped to [minScale, maxScale].
+double absolutePinchScale({
+  required double scaleStart,
+  required double rawScaleAtBaseline,
+  required double rawScale,
+  required double minScale,
+  required double maxScale,
+}) {
+  final double cumulative =
+      rawScaleAtBaseline > 0 ? rawScale / rawScaleAtBaseline : 1.0;
+  return clampDouble(scaleStart * cumulative, minScale, maxScale);
+}