fix: coalesce pinch updates and stop live zoom write-back

Surface Aug6 diag showed sDrop=0 but ~220 same-ms dual ZOOM frames and √2 cur ping-pong from reading currentZoom back into pinch state. Flush once per microtask and embed gitSha in diagnostic meta.

Co-authored-by: Cursor <cursoragent@cursor.com>

lib/editor/canvas/pen_interactive_viewer.dart

@@ -24,6 +24,7 @@
 // because this canvas always uses an infinite boundary, free pan, and no
 // rotation — so that code was provably a no-op here.
 
+import 'dart:async';
 import 'dart:math' as math;
 
 import 'package:flutter/foundation.dart' show clampDouble;
@@ -122,6 +123,12 @@ class _PenInteractiveViewerState extends State<PenInteractiveViewer>
   /// (the reported flicker). Normalizing kills that pop at the source.
   double _rawScaleAtBaseline = 1.0;
 
+  /// Windows ScaleGestureRecognizer emits one onUpdate per finger move in the
+  /// same event-loop turn. Applying both mutates the matrix twice with an
+  /// intermediate state (Surface diag: √2-ish cur ping-pong). Keep latest only.
+  ScaleUpdateDetails? _pendingScaleUpdate;
+  bool _scaleFlushScheduled = false;
+
   // --- Matrix helpers (infinite boundary → no clamping to bounds) -----------
 
   Matrix4 _matrixTranslate(Matrix4 matrix, Offset translation) {
@@ -169,6 +176,8 @@ class _PenInteractiveViewerState extends State<PenInteractiveViewer>
       _scaleAnimation?.removeListener(_handleScaleAnimation);
       _scaleAnimation = null;
     }
+    _pendingScaleUpdate = null;
+    _scaleFlushScheduled = false;
     _gestureType = null;
     _lastPointerCount = details.pointerCount;
     _scaleStart = _transformer.value.getMaxScaleOnAxis();
@@ -178,6 +187,27 @@ class _PenInteractiveViewerState extends State<PenInteractiveViewer>
   }
 
   void _onScaleUpdate(ScaleUpdateDetails details) {
+    // Pointer-count change must apply immediately (re-baseline), not coalesce.
+    if (details.pointerCount != _lastPointerCount) {
+      _pendingScaleUpdate = null;
+      _scaleFlushScheduled = false;
+      _applyScaleUpdate(details);
+      return;
+    }
+    _pendingScaleUpdate = details;
+    if (_scaleFlushScheduled) return;
+    _scaleFlushScheduled = true;
+    scheduleMicrotask(() {
+      _scaleFlushScheduled = false;
+      final pending = _pendingScaleUpdate;
+      _pendingScaleUpdate = null;
+      if (pending != null && mounted && _scaleStart != null) {
+        _applyScaleUpdate(pending);
+      }
+    });
+  }
+
+  void _applyScaleUpdate(ScaleUpdateDetails details) {
     final double scale = _transformer.value.getMaxScaleOnAxis();
     _scaleAnimationFocalPoint = details.localFocalPoint;
 
@@ -220,14 +250,20 @@ class _PenInteractiveViewerState extends State<PenInteractiveViewer>
     final bool focalDrop =
         details.pointerCount >= 2 && focalJumpPx > _kFocalGlitchPx;
 
-    void record(double appliedChange, bool scaleDrop, bool focalDropped) {
+    void record(
+      double currentScale,
+      double appliedChange,
+      bool softClamped,
+      bool focalDropped,
+    ) {
       InputDiagnostics.instance.recordScaleFrame(
         rawScale: details.scale,
         pointerCount: details.pointerCount,
-        currentScale: scale,
+        currentScale: currentScale,
         appliedChange: appliedChange,
         focalJumpPx: focalJumpPx,
-        scaleDrop: scaleDrop,
+        scaleDrop: false,
+        softClamped: softClamped,
         focalDrop: focalDropped,
       );
     }
@@ -252,7 +288,7 @@ class _PenInteractiveViewerState extends State<PenInteractiveViewer>
             _rawScaleAtBaseline = details.scale;
             _lastAppliedScale = step.appliedScale;
           }
-          record(1.0, step.spiked, true);
+          record(_lastAppliedScale, 1.0, step.spiked, true);
           return;
         }
 
@@ -273,7 +309,7 @@ class _PenInteractiveViewerState extends State<PenInteractiveViewer>
         final double applied =
             _lastAppliedScale > 0 ? targetScale / _lastAppliedScale : 1.0;
         _lastAppliedScale = targetScale;
-        record(applied, step.spiked, false);
+        record(targetScale, applied, step.spiked, false);
 
       case _GestureType.pan:
         assert(_referenceFocalPoint != null);
@@ -281,7 +317,7 @@ class _PenInteractiveViewerState extends State<PenInteractiveViewer>
         if (details.scale != 1.0) return;
         if (focalDrop) {
           _referenceFocalPoint = _transformer.toScene(details.localFocalPoint);
-          record(1.0, false, true);
+          record(scale, 1.0, false, true);
           return;
         }
         final Offset translationChange =
@@ -289,11 +325,17 @@ class _PenInteractiveViewerState extends State<PenInteractiveViewer>
         _transformer.value =
             _matrixTranslate(_transformer.value, translationChange);
         _referenceFocalPoint = _transformer.toScene(details.localFocalPoint);
-        record(1.0, false, false);
+        record(scale, 1.0, false, false);
     }
   }
 
   void _onScaleEnd(ScaleEndDetails details) {
+    final pending = _pendingScaleUpdate;
+    _pendingScaleUpdate = null;
+    _scaleFlushScheduled = false;
+    if (pending != null && _scaleStart != null) {
+      _applyScaleUpdate(pending);
+    }
     _scaleStart = null;
     _referenceFocalPoint = null;
     _lastPointerCount = 0;