test: behavioral regression guard for the absolute-snapshot pinch fix (6829076)

Device-independent pinch test over PenCanvas: a steady two-finger spread must
grow the shared transform scale MONOTONICALLY, with no frame popping above the
final scale and snapping back (the flicker invariant the absolute-from-snapshot
rewrite enforces). Plus: a single stylus drag never pans/zooms (the recognizer
excludes stylus) — the transform stays identity while the pen draws.

Locks in 6829076 + the arbiter exclusion as regressions guards without a device.

flutter analyze lib/editor clean; 235/235 tests (+2 widget).

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

test/pen_zoom_widget_test.dart

@@ -0,0 +1,96 @@
+// Behavioral test of the absolute-snapshot pinch fix (6829076) — device-
+// independent (no pdfrx). Simulates a two-finger pinch over PenCanvas and
+// samples the shared transform's scale each frame: a steady spread must grow
+// the scale MONOTONICALLY, never spiking above the final value and snapping
+// back (the reported flicker the fix removes).
+
+import 'package:flutter/gestures.dart';
+import 'package:flutter/material.dart';
+import 'package:flutter_test/flutter_test.dart';
+
+import 'package:badnote/editor/canvas/pen_canvas.dart';
+import 'package:badnote/editor/canvas/pen_stroke.dart';
+
+void main() {
+  const pageSize = Size(400, 600);
+
+  Widget host(TransformationController controller) => MaterialApp(
+        home: Scaffold(
+          body: Center(
+            child: SizedBox(
+              width: pageSize.width,
+              height: pageSize.height,
+              child: PenCanvas(
+                pageWidget: Container(color: const Color(0xFFEEEEEE)),
+                pageSize: pageSize,
+                strokes: const <PenStroke>[],
+                transformationController: controller,
+                tool: CanvasTool.pen,
+                color: const Color(0xFF000000),
+                strokeWidth: 0.004,
+                onStrokeComplete: (_) {},
+                onEraseStroke: (_, __) {},
+              ),
+            ),
+          ),
+        ),
+      );
+
+  testWidgets('a steady two-finger spread grows scale monotonically (no spike)',
+      (tester) async {
+    final controller = TransformationController();
+    addTearDown(controller.dispose);
+    await tester.pumpWidget(host(controller));
+
+    final center = tester.getCenter(find.byType(PenCanvas));
+    final f1 = await tester.startGesture(
+        center + const Offset(-20, 0), kind: PointerDeviceKind.touch);
+    final f2 = await tester.startGesture(
+        center + const Offset(20, 0), kind: PointerDeviceKind.touch);
+
+    final scales = <double>[];
+    for (var i = 0; i < 8; i++) {
+      // Spread the fingers apart symmetrically (pure zoom-in, focal steady).
+      await f1.moveBy(const Offset(-8, 0));
+      await f2.moveBy(const Offset(8, 0));
+      await tester.pump();
+      scales.add(controller.value.getMaxScaleOnAxis());
+    }
+    await f1.up();
+    await f2.up();
+    await tester.pump();
+
+    final maxScale = scales.reduce((a, b) => a > b ? a : b);
+    final finalScale = scales.last;
+
+    // Zoom actually happened.
+    expect(finalScale, greaterThan(1.0));
+    // No frame spiked ABOVE the final scale and snapped back — the fix's
+    // invariant. (Allow a tiny epsilon for fp.)
+    expect(maxScale, lessThanOrEqualTo(finalScale + 1e-6),
+        reason: 'a frame popped bigger than the final scale — flicker');
+    // Monotonic non-decreasing across the steady spread.
+    for (var i = 1; i < scales.length; i++) {
+      expect(scales[i], greaterThanOrEqualTo(scales[i - 1] - 1e-6),
+          reason: 'scale went backwards at frame $i: $scales');
+    }
+  });
+
+  testWidgets('the stylus never pans/zooms (excluded from the recognizer)',
+      (tester) async {
+    final controller = TransformationController();
+    addTearDown(controller.dispose);
+    await tester.pumpWidget(host(controller));
+
+    final center = tester.getCenter(find.byType(PenCanvas));
+    // A single stylus drag must DRAW, not pan — the transform stays identity.
+    final g = await tester.startGesture(center, kind: PointerDeviceKind.stylus);
+    await g.moveBy(const Offset(40, 40));
+    await g.up();
+    await tester.pump();
+
+    expect(controller.value.getMaxScaleOnAxis(), closeTo(1.0, 1e-9));
+    expect(controller.value.getTranslation().x, closeTo(0, 1e-6));
+    expect(controller.value.getTranslation().y, closeTo(0, 1e-6));
+  });
+}