Fix a rotation gesture bug (#79035)

packages/flutter/lib/src/gestures/scale.dart

@@ -441,7 +441,7 @@ class ScaleGestureRecognizer extends OneSequenceGestureRecognizer {
         pointerEndId: _pointerQueue[1],
         pointerEndLocation: _pointerLocations[_pointerQueue[1]]!,
       );
-      _currentLine = null;
+      _currentLine = _initialLine;
     }
   }
 

packages/flutter/test/gestures/scale_test.dart

@@ -572,6 +572,34 @@ void main() {
     tap.dispose();
   });
 
+  // Regressing test for https://github.com/flutter/flutter/issues/78941
+  testGesture('First rotation test', (GestureTester tester) {
+    final ScaleGestureRecognizer scale = ScaleGestureRecognizer();
+
+    double? updatedRotation;
+    scale.onUpdate = (ScaleUpdateDetails details) {
+      updatedRotation = details.rotation;
+    };
+
+    final TestPointer pointer1 = TestPointer(1);
+    final PointerDownEvent down = pointer1.down(Offset.zero);
+    scale.addPointer(down);
+    tester.closeArena(1);
+    tester.route(down);
+
+    final TestPointer pointer2 = TestPointer(2);
+    final PointerDownEvent down2 = pointer2.down(const Offset(10.0, 10.0));
+    scale.addPointer(down2);
+    tester.closeArena(2);
+    tester.route(down2);
+
+    expect(updatedRotation, isNull);
+
+    // Rotation 45°.
+    tester.route(pointer2.move(const Offset(0.0, 10.0)));
+    expect(updatedRotation, math.pi / 4.0);
+  });
+
   testGesture('Scale gestures pointer count test', (GestureTester tester) {
     final ScaleGestureRecognizer scale = ScaleGestureRecognizer();