fix: soft-clamp pinch zoom and Krita-inspired brush opacity

Hard SDROP avalanches froze lastRaw while zoom still crawled; soft-clamp
and re-anchor instead. Ballpoint is near-solid, pencil uses soft √p without
multiply stacking; PDF ink falls back to nearest page during zoom settle.

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

lib/editor/canvas/pen_editor_screen.dart

@@ -169,7 +169,6 @@ class _PenEditorScreenState extends State<PenEditorScreen> {
   /// is a Windows multi-touch glitch and is dropped (so the zoom can't pop).
   /// Logs showed ~1.30 spikes — keep the band below that.
   static const double _kScaleGlitchHi = 1.18;
-  static const double _kScaleGlitchLo = 1 / _kScaleGlitchHi;
 
   /// A single-frame focal-midpoint jump beyond this is a touch misread → drop.
   static const double _kFocalGlitchPx = 100.0;
@@ -181,9 +180,6 @@ class _PenEditorScreenState extends State<PenEditorScreen> {
   /// Pointer count of the previous accepted pinch frame; a change re-baselines.
   int _pinchPointerCount = 0;
 
-  /// The recognizer's cumulative `details.scale` on the previous accepted frame.
-  double _pinchLastRawScale = 1.0;
-
   /// The absolute scale we last APPLIED. Re-baseline anchors to THIS (not a live
   /// matrix read) so the displayed scale stays continuous across a finger blip.
   double _pinchLastAppliedScale = 1.0;
@@ -575,14 +571,16 @@ class _PenEditorScreenState extends State<PenEditorScreen> {
     return null;
   }
 
-  /// Map a global pen position to (pageIndex, normalized-in-page) using the
-  /// controller's document-space page layout rects. Returns null if outside
-  /// every page box or the viewer isn't ready.
+  /// Map a global pen position to (pageIndex, normalized-in-page). During a
+  /// zoom glitch `globalToDocument` can miss every page rect — fall back to the
+  /// nearest page so strokes do not silently vanish mid-gesture.
   ({int page, Offset normalized})? _documentToPage(Offset global) {
     if (!_controller.isReady) return null;
     final doc = _controller.globalToDocument(global);
     if (doc == null) return null;
     final rects = _controller.layout.pageLayouts;
+    if (rects.isEmpty) return null;
+
     for (var i = 0; i < rects.length; i++) {
       final r = rects[i];
       if (r.contains(doc)) {
@@ -591,7 +589,24 @@ class _PenEditorScreenState extends State<PenEditorScreen> {
         return (page: i, normalized: Offset(nx, ny));
       }
     }
-    return null;
+
+    // Nearest-page fallback (common while pinch is settling).
+    var bestI = 0;
+    var bestDist = double.infinity;
+    for (var i = 0; i < rects.length; i++) {
+      final r = rects[i];
+      final cx = doc.dx.clamp(r.left, r.right);
+      final cy = doc.dy.clamp(r.top, r.bottom);
+      final d = (Offset(cx, cy) - doc).distanceSquared;
+      if (d < bestDist) {
+        bestDist = d;
+        bestI = i;
+      }
+    }
+    final r = rects[bestI];
+    final nx = ((doc.dx - r.left) / r.width).clamp(0.0, 1.0);
+    final ny = ((doc.dy - r.top) / r.height).clamp(0.0, 1.0);
+    return (page: bestI, normalized: Offset(nx, ny));
   }
 
   /// Hit-test scratch-link markers near [normalized] on [page].
@@ -894,7 +909,6 @@ class _PenEditorScreenState extends State<PenEditorScreen> {
     if (!_controller.isReady) return;
     _pinchScaleStart = _controller.currentZoom;
     _pinchPointerCount = details.pointerCount;
-    _pinchLastRawScale = 1.0;
     _pinchLastAppliedScale = _pinchScaleStart!;
     _pinchRawScaleAtBaseline = 1.0;
   }
@@ -909,63 +923,73 @@ class _PenEditorScreenState extends State<PenEditorScreen> {
     if (details.pointerCount != _pinchPointerCount) {
       _pinchPointerCount = details.pointerCount;
       _pinchScaleStart = _pinchLastAppliedScale;
-      _pinchLastRawScale = details.scale;
       _pinchRawScaleAtBaseline = details.scale;
+      InputDiagnostics.instance.recordRebaseline();
       return;
     }
 
-    // Per-frame finger-motion ratio from the recognizer's OWN cumulative scale.
-    // A ratio outside the glitch band is a multi-touch spike → drop the frame;
-    // absolute tracking means the next good frame resumes from the true span.
-    final rawRatio =
-        _pinchLastRawScale > 0 ? details.scale / _pinchLastRawScale : 1.0;
-    final scaleDrop =
-        rawRatio > _kScaleGlitchHi || rawRatio < _kScaleGlitchLo;
+    // Soft-clamp per-step change (Surface diag: hard SDROP avalanche when
+    // lastRaw froze while live zoom still crawled). Always apply + advance.
+    final step = softClampedPinchStep(
+      scaleStart: _pinchScaleStart!,
+      rawScaleAtBaseline: _pinchRawScaleAtBaseline,
+      rawScale: details.scale,
+      lastAppliedScale: _pinchLastAppliedScale,
+      minScale: _kPinchMinScale,
+      maxScale: _kPinchMaxScale,
+      maxStepRatio: _kScaleGlitchHi,
+    );
     final focalDrop = details.focalPointDelta.distance > _kFocalGlitchPx;
-    if (scaleDrop || focalDrop) {
+    if (focalDrop) {
+      // Keep scale continuous; only skip the focal jump this frame.
+      if (step.reanchor) {
+        _pinchScaleStart = step.appliedScale;
+        _pinchRawScaleAtBaseline = details.scale;
+        _pinchLastAppliedScale = step.appliedScale;
+      }
       InputDiagnostics.instance.recordScaleFrame(
         rawScale: details.scale,
         pointerCount: details.pointerCount,
         currentScale: _pinchLastAppliedScale,
         appliedChange: 1.0,
         focalJumpPx: details.focalPointDelta.distance,
-        scaleDrop: scaleDrop,
-        focalDrop: focalDrop,
+        scaleDrop: step.spiked,
+        focalDrop: true,
       );
       return;
     }
 
-    final targetScale = absolutePinchScale(
-      scaleStart: _pinchScaleStart!,
-      rawScaleAtBaseline: _pinchRawScaleAtBaseline,
-      rawScale: details.scale,
-      minScale: _kPinchMinScale,
-      maxScale: _kPinchMaxScale,
-    );
+    final targetScale = step.appliedScale;
+    if (step.reanchor) {
+      _pinchScaleStart = targetScale;
+      _pinchRawScaleAtBaseline = details.scale;
+    }
 
-    // Focal zoom: keep the document point under the live focal (finger midpoint)
-    // fixed, which also yields 2-finger pan for free as the focal moves.
-    // localFocalPoint is in the viewer's local coords (the overlay fills it).
     _controller.zoomOnLocalPosition(
       localPosition: details.localFocalPoint,
       newZoom: targetScale,
       duration: Duration.zero,
     );
 
-    final applied =
-        _pinchLastAppliedScale > 0 ? targetScale / _pinchLastAppliedScale : 1.0;
+    // Prefer controller read-back so we stay locked to what pdfrx actually
+    // applied (guards against a second consumer nudging zoom).
+    final live = _controller.currentZoom;
+    final appliedScale = live > 0 ? live : targetScale;
+
+    final applied = _pinchLastAppliedScale > 0
+        ? appliedScale / _pinchLastAppliedScale
+        : 1.0;
     InputDiagnostics.instance.recordScaleFrame(
       rawScale: details.scale,
       pointerCount: details.pointerCount,
-      currentScale: targetScale,
+      currentScale: appliedScale,
       appliedChange: applied,
       focalJumpPx: details.focalPointDelta.distance,
-      scaleDrop: false,
+      scaleDrop: step.spiked,
       focalDrop: false,
     );
 
-    _pinchLastRawScale = details.scale;
-    _pinchLastAppliedScale = targetScale;
+    _pinchLastAppliedScale = appliedScale;
   }
 
   void _onPinchEnd(ScaleEndDetails details) {

lib/editor/canvas/pen_interactive_viewer.dart

@@ -48,7 +48,6 @@ const Set<PointerDeviceKind> _kPanZoomDevices = <PointerDeviceKind>{
 /// intent — that frame is dropped so the zoom can't pop and snap back.
 /// Device logs showed spikes ~1.30; keep the band under that so jumps die.
 const double _kScaleGlitchHi = 1.18;
-const double _kScaleGlitchLo = 1 / _kScaleGlitchHi;
 
 /// During a 2-finger gesture the focal point (finger midpoint) should move
 /// smoothly. A single-frame local jump beyond this is a Windows touch misread,
@@ -107,15 +106,8 @@ class _PenInteractiveViewerState extends State<PenInteractiveViewer>
   /// applying a frame whose scale/focal still refer to the old finger set.
   int _lastPointerCount = 0;
 
-  /// The recognizer's cumulative `details.scale` and the absolute scale we last
-  /// APPLIED, both as of the previous accepted frame. The pinch is driven
-  /// absolutely from these + the gesture-start snapshot — we never read the live
-  /// matrix back into the per-frame scale change. (Re-reading
-  /// `getMaxScaleOnAxis()` per frame was the flicker source: a single transient
-  /// mis-read/interleaved write made `desiredScale/liveScale` demand a ~1.3–1.4x
-  /// jump for one frame and snap back. The glitch guard missed it because the
-  /// spike sat just under the 1.4 threshold.)
-  double _lastRawScale = 1.0;
+  /// The absolute scale we last APPLIED. Soft-clamp limits the step from this
+  /// value; we never read the live matrix back into the per-frame scale change.
   double _lastAppliedScale = 1.0;
 
   /// The recognizer's cumulative `details.scale` AT THE CURRENT BASELINE (the
@@ -181,7 +173,6 @@ class _PenInteractiveViewerState extends State<PenInteractiveViewer>
     _lastPointerCount = details.pointerCount;
     _scaleStart = _transformer.value.getMaxScaleOnAxis();
     _referenceFocalPoint = _transformer.toScene(details.localFocalPoint);
-    _lastRawScale = 1.0;
     _lastAppliedScale = _scaleStart!;
     _rawScaleAtBaseline = 1.0;
   }
@@ -206,7 +197,6 @@ class _PenInteractiveViewerState extends State<PenInteractiveViewer>
       // any transient in the live matrix.
       _scaleStart = _lastAppliedScale;
       _referenceFocalPoint = _transformer.toScene(details.localFocalPoint);
-      _lastRawScale = details.scale;
       // Re-anchor the cumulative scale to THIS frame's details.scale so the next
       // good frame resumes from _scaleStart (not _scaleStart × a stale ratio).
       _rawScaleAtBaseline = details.scale;
@@ -245,39 +235,32 @@ class _PenInteractiveViewerState extends State<PenInteractiveViewer>
     switch (_gestureType!) {
       case _GestureType.scale:
         assert(_scaleStart != null);
-        // Per-frame finger-motion ratio from the recognizer's OWN cumulative
-        // scale — the clean, monotonic signal (verified against device logs).
-        // Crucially we do NOT divide by the live matrix scale here: feeding
-        // getMaxScaleOnAxis() back in is what let a single mis-read pop the zoom
-        // and snap back. A ratio outside the glitch band is a real multi-touch
-        // spike → drop the frame; absolute tracking means the next good frame
-        // resumes from the true finger span, so the spike never shows.
-        final double rawRatio =
-            _lastRawScale > 0 ? details.scale / _lastRawScale : 1.0;
-        final bool scaleDrop =
-            rawRatio > _kScaleGlitchHi || rawRatio < _kScaleGlitchLo;
-        if (scaleDrop || focalDrop) {
-          record(1.0, scaleDrop, focalDrop);
-          return;
-        }
-
-        // Drive the transform ABSOLUTELY from the gesture-start snapshot: the
-        // target scale is `_scaleStart * details.scale`, and we re-anchor so the
-        // scene point that was under the focal at gesture start stays under the
-        // CURRENT focal (which also yields 2-finger pan for free). Closed form
-        // 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.
-        // 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(
+        // Soft-clamp per-step change instead of hard-dropping (Surface diag:
+        // hard SDROP froze lastRaw and avalanched while the matrix still moved).
+        final SoftPinchStep step = softClampedPinchStep(
           scaleStart: _scaleStart!,
           rawScaleAtBaseline: _rawScaleAtBaseline,
           rawScale: details.scale,
+          lastAppliedScale: _lastAppliedScale,
           minScale: widget.minScale,
           maxScale: widget.maxScale,
+          maxStepRatio: _kScaleGlitchHi,
         );
+        if (focalDrop) {
+          if (step.reanchor) {
+            _scaleStart = step.appliedScale;
+            _rawScaleAtBaseline = details.scale;
+            _lastAppliedScale = step.appliedScale;
+          }
+          record(1.0, step.spiked, true);
+          return;
+        }
+
+        final double targetScale = step.appliedScale;
+        if (step.reanchor) {
+          _scaleStart = targetScale;
+          _rawScaleAtBaseline = details.scale;
+        }
         final Offset focal = details.localFocalPoint;
         final double tx = focal.dx - targetScale * _referenceFocalPoint!.dx;
         final double ty = focal.dy - targetScale * _referenceFocalPoint!.dy;
@@ -289,9 +272,8 @@ class _PenInteractiveViewerState extends State<PenInteractiveViewer>
 
         final double applied =
             _lastAppliedScale > 0 ? targetScale / _lastAppliedScale : 1.0;
-        _lastRawScale = details.scale;
         _lastAppliedScale = targetScale;
-        record(applied, false, false);
+        record(applied, step.spiked, false);
 
       case _GestureType.pan:
         assert(_referenceFocalPoint != null);

lib/editor/canvas/pinch_scale_solver.dart

@@ -18,6 +18,12 @@
 // 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.
+//
+// Soft-clamp (Surface 2026-08-05 diag): a HARD drop of frames whose per-step
+// ratio exceeds the glitch band caused an avalanche — lastRaw never advanced,
+// so every subsequent frame also dropped while pdfrx/live zoom still crawled.
+// [softClampedPinchStep] always returns an applied scale, clamping the step,
+// and tells the caller to re-anchor when a spike was clipped.
 
 import 'package:flutter/foundation.dart' show clampDouble;
 
@@ -39,3 +45,59 @@ double absolutePinchScale({
       rawScaleAtBaseline > 0 ? rawScale / rawScaleAtBaseline : 1.0;
   return clampDouble(scaleStart * cumulative, minScale, maxScale);
 }
+
+/// Result of one soft-clamped pinch step.
+class SoftPinchStep {
+  const SoftPinchStep({
+    required this.appliedScale,
+    required this.reanchor,
+    required this.spiked,
+  });
+
+  /// Scale to write into the matrix / controller this frame.
+  final double appliedScale;
+
+  /// When true the caller must set `scaleStart = appliedScale` and
+  /// `rawScaleAtBaseline = rawScale` so absolute tracking does not keep
+  /// fighting the clamp on later frames.
+  final bool reanchor;
+
+  /// True when the ideal absolute target was clipped by the per-step band.
+  final bool spiked;
+}
+
+/// Soft-clamp the per-frame scale change instead of dropping the frame.
+///
+/// Ideal scale comes from [absolutePinchScale]. The step from
+/// [lastAppliedScale] is then limited to `[1/maxStepRatio, maxStepRatio]`.
+/// Spikes still get partially applied (smooth catch-up) and the caller
+/// re-anchors so the next frame starts clean.
+SoftPinchStep softClampedPinchStep({
+  required double scaleStart,
+  required double rawScaleAtBaseline,
+  required double rawScale,
+  required double lastAppliedScale,
+  required double minScale,
+  required double maxScale,
+  required double maxStepRatio,
+}) {
+  final ideal = absolutePinchScale(
+    scaleStart: scaleStart,
+    rawScaleAtBaseline: rawScaleAtBaseline,
+    rawScale: rawScale,
+    minScale: minScale,
+    maxScale: maxScale,
+  );
+  if (lastAppliedScale <= 0 || maxStepRatio <= 1.0) {
+    return SoftPinchStep(appliedScale: ideal, reanchor: false, spiked: false);
+  }
+  final lo = lastAppliedScale / maxStepRatio;
+  final hi = lastAppliedScale * maxStepRatio;
+  final applied = clampDouble(ideal, lo, hi);
+  final spiked = applied != ideal;
+  return SoftPinchStep(
+    appliedScale: clampDouble(applied, minScale, maxScale),
+    reanchor: spiked,
+    spiked: spiked,
+  );
+}