diff --git a/lib/web_ui/lib/src/engine/pointer_binding.dart b/lib/web_ui/lib/src/engine/pointer_binding.dart
index dc116ab9f3b90..21fc00c81729b 100644
--- a/lib/web_ui/lib/src/engine/pointer_binding.dart
+++ b/lib/web_ui/lib/src/engine/pointer_binding.dart
@@ -33,8 +33,9 @@ typedef _PointerDataCallback = void Function(Iterable<ui.PointerData>);
 // here, we use an already very large number (30 bits).
 const int _kButtonsMask = 0x3FFFFFFF;
 
-// Intentionally set to -1 so it doesn't conflict with other device IDs.
+// Intentionally set to -1 or -2 so it doesn't conflict with other device IDs.
 const int _mouseDeviceId = -1;
+const int _trackpadDeviceId = -2;
 
 const int _kPrimaryMouseButton = 0x1;
 const int _kSecondaryMouseButton = 0x2;
@@ -421,8 +422,10 @@ mixin _WheelEventListenerMixin on _BaseAdapter {
     const int domDeltaPage = 0x02;
 
     ui.PointerDeviceKind kind = ui.PointerDeviceKind.mouse;
+    int deviceId = _mouseDeviceId;
     if (_isTrackpadEvent(event)) {
       kind = ui.PointerDeviceKind.trackpad;
+      deviceId = _trackpadDeviceId;
     }
 
     // Flutter only supports pixel scroll delta. Convert deltaMode values
@@ -459,7 +462,7 @@ mixin _WheelEventListenerMixin on _BaseAdapter {
       timeStamp: _BaseAdapter._eventTimeStampToDuration(event.timeStamp!),
       kind: kind,
       signalKind: ui.PointerSignalKind.scroll,
-      device: _mouseDeviceId,
+      device: deviceId,
       physicalX: offset.dx * ui.window.devicePixelRatio,
       physicalY: offset.dy * ui.window.devicePixelRatio,
       buttons: event.buttons!.toInt(),
diff --git a/lib/web_ui/test/engine/pointer_binding_test.dart b/lib/web_ui/test/engine/pointer_binding_test.dart
index 6bba0cf5ce080..2dedf3ea5f491 100644
--- a/lib/web_ui/test/engine/pointer_binding_test.dart
+++ b/lib/web_ui/test/engine/pointer_binding_test.dart
@@ -1265,6 +1265,7 @@ void testMain() {
           packets[0].data[1].signalKind, equals(ui.PointerSignalKind.scroll));
       expect(
           packets[0].data[1].kind, equals(ui.PointerDeviceKind.trackpad));
+      expect(packets[0].data[1].device, equals(-2));
       expect(packets[0].data[1].pointerIdentifier, equals(0));
       expect(packets[0].data[1].synthesized, isFalse);
       expect(packets[0].data[1].physicalX, equals(10.0 * dpi));
@@ -1281,6 +1282,7 @@ void testMain() {
           packets[1].data[0].signalKind, equals(ui.PointerSignalKind.scroll));
       expect(
           packets[1].data[0].kind, equals(ui.PointerDeviceKind.trackpad));
+      expect(packets[1].data[0].device, equals(-2));
       expect(packets[1].data[0].pointerIdentifier, equals(0));
       expect(packets[1].data[0].synthesized, isFalse);
       expect(packets[1].data[0].physicalX, equals(10.0 * dpi));
@@ -1297,6 +1299,7 @@ void testMain() {
           packets[2].data[0].signalKind, equals(ui.PointerSignalKind.scroll));
       expect(
           packets[2].data[0].kind, equals(ui.PointerDeviceKind.trackpad));
+      expect(packets[2].data[0].device, equals(-2));
       expect(packets[2].data[0].pointerIdentifier, equals(0));
       expect(packets[2].data[0].synthesized, isFalse);
       expect(packets[2].data[0].physicalX, equals(10.0 * dpi));
@@ -1313,6 +1316,7 @@ void testMain() {
           packets[3].data[0].signalKind, equals(ui.PointerSignalKind.scroll));
       expect(
           packets[3].data[0].kind, equals(ui.PointerDeviceKind.trackpad));
+      expect(packets[3].data[0].device, equals(-2));
       expect(packets[3].data[0].pointerIdentifier, equals(0));
       expect(packets[3].data[0].synthesized, isFalse);
       expect(packets[3].data[0].physicalX, equals(10.0 * dpi));
@@ -1323,13 +1327,14 @@ void testMain() {
       expect(packets[3].data[0].scrollDeltaY, equals(119.0));
 
       // Because the delta is in increments of 120, and is not similar to the
-      // previous event, but occured soon after the previous event, it will be
+      // previous event, but occurred soon after the previous event, it will be
       // a trackpad event.
       expect(packets[4].data[0].change, equals(ui.PointerChange.hover));
       expect(
           packets[4].data[0].signalKind, equals(ui.PointerSignalKind.scroll));
       expect(
           packets[4].data[0].kind, equals(ui.PointerDeviceKind.trackpad));
+      expect(packets[4].data[0].device, equals(-2));
       expect(packets[4].data[0].pointerIdentifier, equals(0));
       expect(packets[4].data[0].synthesized, isFalse);
       expect(packets[4].data[0].physicalX, equals(10.0 * dpi));
@@ -1339,22 +1344,39 @@ void testMain() {
       expect(packets[4].data[0].scrollDeltaX, equals(-120.0));
       expect(packets[4].data[0].scrollDeltaY, equals(-120.0));
 
-      // Because the delta is in increments of 120, and is not similar to
-      // the previous event, and occured long after the previous event, it will be a mouse event.
-      expect(packets[5].data, hasLength(1));
-      expect(packets[5].data[0].change, equals(ui.PointerChange.hover));
+      // An add will be synthesized.
+      expect(packets[5].data, hasLength(2));
+      expect(packets[5].data[0].change, equals(ui.PointerChange.add));
       expect(
-          packets[5].data[0].signalKind, equals(ui.PointerSignalKind.scroll));
+          packets[5].data[0].signalKind, equals(ui.PointerSignalKind.none));
       expect(
           packets[5].data[0].kind, equals(ui.PointerDeviceKind.mouse));
+      expect(packets[5].data[0].device, equals(-1));
       expect(packets[5].data[0].pointerIdentifier, equals(0));
-      expect(packets[5].data[0].synthesized, isFalse);
+      expect(packets[5].data[0].synthesized, isTrue);
       expect(packets[5].data[0].physicalX, equals(10.0 * dpi));
       expect(packets[5].data[0].physicalY, equals(10.0 * dpi));
       expect(packets[5].data[0].physicalDeltaX, equals(0.0));
       expect(packets[5].data[0].physicalDeltaY, equals(0.0));
       expect(packets[5].data[0].scrollDeltaX, equals(0.0));
       expect(packets[5].data[0].scrollDeltaY, equals(-120.0));
+      // Because the delta is in increments of 120, and is not similar to
+      // the previous event, and occurred long after the previous event, it will
+      // be a mouse event.
+      expect(packets[5].data[1].change, equals(ui.PointerChange.hover));
+      expect(
+          packets[5].data[1].signalKind, equals(ui.PointerSignalKind.scroll));
+      expect(
+          packets[5].data[1].kind, equals(ui.PointerDeviceKind.mouse));
+      expect(packets[5].data[1].device, equals(-1));
+      expect(packets[5].data[1].pointerIdentifier, equals(0));
+      expect(packets[5].data[1].synthesized, isFalse);
+      expect(packets[5].data[1].physicalX, equals(10.0 * dpi));
+      expect(packets[5].data[1].physicalY, equals(10.0 * dpi));
+      expect(packets[5].data[1].physicalDeltaX, equals(0.0));
+      expect(packets[5].data[1].physicalDeltaY, equals(0.0));
+      expect(packets[5].data[1].scrollDeltaX, equals(0.0));
+      expect(packets[5].data[1].scrollDeltaY, equals(-120.0));
 
       // Because the delta is not in increments of 120 and has non-matching
       // wheelDelta, it will be a mouse event.
@@ -1364,6 +1386,7 @@ void testMain() {
           packets[6].data[0].signalKind, equals(ui.PointerSignalKind.scroll));
       expect(
           packets[6].data[0].kind, equals(ui.PointerDeviceKind.mouse));
+      expect(packets[6].data[0].device, equals(-1));
       expect(packets[6].data[0].pointerIdentifier, equals(0));
       expect(packets[6].data[0].synthesized, isFalse);
       expect(packets[6].data[0].physicalX, equals(10.0 * dpi));