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/input_diagnostics.dart

@@ -16,7 +16,8 @@ class InputDiagnostics extends ChangeNotifier {
   static final InputDiagnostics instance = InputDiagnostics._();
 
   int frames = 0;
-  int scaleDropped = 0; // frames rejected as a scale glitch
+  int scaleDropped = 0; // frames HARD-rejected (legacy; prefer soft-clamp)
+  int softClamped = 0; // frames whose step was soft-clamped (still applied)
   int focalDropped = 0; // frames rejected as a focal/position glitch
   int rebaselines = 0; // pointer-count re-baselines
   int pointerCountMax = 0;
@@ -42,13 +43,16 @@ class InputDiagnostics extends ChangeNotifier {
     required double focalJumpPx,
     required bool scaleDrop,
     required bool focalDrop,
+    bool softClamped = false,
+    double? liveScale,
   }) {
     frames++;
     if (scaleDrop) scaleDropped++;
+    if (softClamped) this.softClamped++;
     if (focalDrop) focalDropped++;
     if (rawScale < rawScaleMin) rawScaleMin = rawScale;
     if (rawScale > rawScaleMax) rawScaleMax = rawScale;
-    final double resulting = currentScale * appliedChange;
+    final double resulting = currentScale;
     if (resulting < scaleMin) scaleMin = resulting;
     if (resulting > scaleMax) scaleMax = resulting;
     if (pointerCount > pointerCountMax) pointerCountMax = pointerCount;
@@ -56,16 +60,21 @@ class InputDiagnostics extends ChangeNotifier {
     final double jump = appliedChange >= 1 ? appliedChange : 1 / appliedChange;
     if (jump > maxAppliedScaleJump) maxAppliedScaleJump = jump;
 
+    final liveBit = liveScale == null
+        ? ''
+        : ' live=${liveScale.toStringAsFixed(3)}';
     final String line = 'p$pointerCount raw=${rawScale.toStringAsFixed(3)} '
         'ch=${appliedChange.toStringAsFixed(3)} '
-        'cur=${currentScale.toStringAsFixed(3)} '
+        'cur=${currentScale.toStringAsFixed(3)}$liveBit '
         'fj=${focalJumpPx.toStringAsFixed(0)}'
-        '${scaleDrop ? " SDROP" : ""}${focalDrop ? " FDROP" : ""}';
+        '${softClamped ? " SCLAMP" : ""}'
+        '${scaleDrop ? " SDROP" : ""}'
+        '${focalDrop ? " FDROP" : ""}';
     _trace.add(line);
     if (_trace.length > 24) _trace.removeAt(0);
     FrameSampler.instance.recordZoom(
       rawScale: rawScale,
-      scaleDrop: scaleDrop,
+      scaleDrop: scaleDrop || softClamped,
       focalDrop: focalDrop,
       focalJumpPx: focalJumpPx,
     );
@@ -74,7 +83,8 @@ class InputDiagnostics extends ChangeNotifier {
   }
 
   void reset() {
-    frames = scaleDropped = focalDropped = rebaselines = pointerCountMax = 0;
+    frames = scaleDropped = softClamped = focalDropped = rebaselines =
+        pointerCountMax = 0;
     rawScaleMin = scaleMin = double.infinity;
     rawScaleMax = scaleMax = 0;
     maxFocalJumpPx = 0;
@@ -83,15 +93,15 @@ class InputDiagnostics extends ChangeNotifier {
     notifyListeners();
   }
 
-  String _f(double v) => v.isFinite ? v.toStringAsFixed(2) : '-';
-
   String summary() {
-    if (frames == 0) return 'zoom: (pinch to record)';
-    return 'zoom f=$frames sDrop=$scaleDropped fDrop=$focalDropped '
-        'rebase=$rebaselines pMax=$pointerCountMax\n'
-        '  raw=${_f(rawScaleMin)}..${_f(rawScaleMax)} '
-        'scale=${_f(scaleMin)}..${_f(scaleMax)}\n'
+    final rawLo = rawScaleMin.isFinite ? rawScaleMin.toStringAsFixed(2) : '-';
+    final rawHi = rawScaleMax > 0 ? rawScaleMax.toStringAsFixed(2) : '-';
+    final scLo = scaleMin.isFinite ? scaleMin.toStringAsFixed(2) : '-';
+    final scHi = scaleMax > 0 ? scaleMax.toStringAsFixed(2) : '-';
+    return 'zoom f=$frames sDrop=$scaleDropped sClamp=$softClamped '
+        'fDrop=$focalDropped rebase=$rebaselines pMax=$pointerCountMax\n'
+        '  raw=$rawLo..$rawHi scale=$scLo..$scHi\n'
         '  maxFocalJump=${maxFocalJumpPx.toStringAsFixed(0)}px '
-        'maxScaleJump=${_f(maxAppliedScaleJump)}';
+        'maxScaleJump=${maxAppliedScaleJump.toStringAsFixed(2)}';
   }
 }