[web] Work around wrong pointerId in coalesced events in iOS Safari 18.2 (flutter/engine#56719)

In iOS 18.2, Safari [added support](https://developer.apple.com/documentation/safari-release-notes/safari-18_2-release-notes#Web-API) for the [`getCoalescedEvents`](https://developer.mozilla.org/en-US/docs/Web/API/PointerEvent/getCoalescedEvents) API.

That being said, the API seems to be incomplete (or at least doesn't match other browsers' behavior). The coalesced events lack a [`pointerId`](https://developer.mozilla.org/en-US/docs/Web/API/PointerEvent/pointerId) and [`target`](https://developer.mozilla.org/en-US/docs/Web/API/Event/target) properties.

I'm not sure if this issue will be fixed in the stable release of iOS 18.2, so in the meantime, this PR implements a workaround to avoid this issue.

Fixes https://github.com/flutter/flutter/issues/158299
Fixes https://github.com/flutter/flutter/issues/155987

engine/src/flutter/lib/web_ui/lib/src/engine/pointer_binding.dart

@@ -1039,20 +1039,32 @@ class _PointerAdapter extends _BaseAdapter with _WheelEventListenerMixin {
     //
     // TODO(dkwingsmt): Investigate whether we can configure the behavior for
     // `_viewTarget`. https://github.com/flutter/flutter/issues/157968
-    _addPointerEventListener(_globalTarget, 'pointermove', (DomPointerEvent event) {
-      final int device = _getPointerId(event);
+    _addPointerEventListener(_globalTarget, 'pointermove', (DomPointerEvent moveEvent) {
+      final int device = _getPointerId(moveEvent);
       final _ButtonSanitizer sanitizer = _ensureSanitizer(device);
       final List<ui.PointerData> pointerData = <ui.PointerData>[];
-      final List<DomPointerEvent> expandedEvents = _expandEvents(event);
+      final List<DomPointerEvent> expandedEvents = _expandEvents(moveEvent);
       for (final DomPointerEvent event in expandedEvents) {
         final _SanitizedDetails? up = sanitizer.sanitizeMissingRightClickUp(buttons: event.buttons!.toInt());
         if (up != null) {
-          _convertEventsToPointerData(data: pointerData, event: event, details: up);
+          _convertEventsToPointerData(
+            data: pointerData,
+            event: event,
+            details: up,
+            pointerId: device,
+            eventTarget: moveEvent.target,
+          );
         }
         final _SanitizedDetails move = sanitizer.sanitizeMoveEvent(buttons: event.buttons!.toInt());
-        _convertEventsToPointerData(data: pointerData, event: event, details: move);
+        _convertEventsToPointerData(
+          data: pointerData,
+          event: event,
+          details: move,
+          pointerId: device,
+          eventTarget: moveEvent.target,
+        );
       }
-      _callback(event, pointerData);
+      _callback(moveEvent, pointerData);
     });
 
     _addPointerEventListener(_viewTarget, 'pointerleave', (DomPointerEvent event) {
@@ -1106,12 +1118,17 @@ class _PointerAdapter extends _BaseAdapter with _WheelEventListenerMixin {
     required List<ui.PointerData> data,
     required DomPointerEvent event,
     required _SanitizedDetails details,
+    // `pointerId` and `eventTarget` are optional but useful when it's not
+    // desired to get those values from the event object. For example, when the
+    // event is a coalesced event.
+    int? pointerId,
+    DomEventTarget? eventTarget,
   }) {
     final ui.PointerDeviceKind kind = _pointerTypeToDeviceKind(event.pointerType!);
     final double tilt = _computeHighestTilt(event);
     final Duration timeStamp = _BaseAdapter._eventTimeStampToDuration(event.timeStamp!);
     final num? pressure = event.pressure;
-    final ui.Offset offset = computeEventOffsetToTarget(event, _view);
+    final ui.Offset offset = computeEventOffsetToTarget(event, _view, eventTarget: eventTarget);
     _pointerDataConverter.convert(
       data,
       viewId: _view.viewId,
@@ -1119,7 +1136,7 @@ class _PointerAdapter extends _BaseAdapter with _WheelEventListenerMixin {
       timeStamp: timeStamp,
       kind: kind,
       signalKind: ui.PointerSignalKind.none,
-      device: _getPointerId(event),
+      device: pointerId ?? _getPointerId(event),
       physicalX: offset.dx * _view.devicePixelRatio,
       physicalY: offset.dy * _view.devicePixelRatio,
       buttons: details.buttons,

engine/src/flutter/lib/web_ui/lib/src/engine/pointer_binding/event_position_helper.dart

@@ -12,18 +12,23 @@ import '../text_editing/text_editing.dart';
 import '../vector_math.dart';
 import '../window.dart';
 
-/// Returns an [ui.Offset] of the position of [event], relative to the position of [actualTarget].
+/// Returns an [ui.Offset] of the position of [event], relative to the position
+/// of the Flutter [view].
 ///
 /// The offset is *not* multiplied by DPR or anything else, it's the closest
 /// to what the DOM would return if we had currentTarget readily available.
 ///
-/// This needs an `actualTarget`, because the `event.currentTarget` (which is what
-/// this would really need to use) gets lost when the `event` comes from a "coalesced"
-/// event.
+/// This needs an `eventTarget`, because the `event.target` (which is what
+/// this would really need to use) gets lost when the `event` comes from a
+/// "coalesced" event (see https://github.com/flutter/flutter/issues/155987).
 ///
 /// It also takes into account semantics being enabled to fix the case where
 /// offsetX, offsetY == 0 (TalkBack events).
-ui.Offset computeEventOffsetToTarget(DomMouseEvent event, EngineFlutterView view) {
+ui.Offset computeEventOffsetToTarget(
+  DomMouseEvent event,
+  EngineFlutterView view, {
+  DomEventTarget? eventTarget,
+}) {
   final DomElement actualTarget = view.dom.rootElement;
   // On a TalkBack event
   if (EngineSemantics.instance.semanticsEnabled && event.offsetX == 0 && event.offsetY == 0) {
@@ -31,7 +36,8 @@ ui.Offset computeEventOffsetToTarget(DomMouseEvent event, EngineFlutterView view
   }
 
   // On one of our text-editing nodes
-  final bool isInput = view.dom.textEditingHost.contains(event.target! as DomNode);
+  eventTarget ??= event.target!;
+  final bool isInput = view.dom.textEditingHost.contains(eventTarget as DomNode);
   if (isInput) {
     final EditableTextGeometry? inputGeometry = textEditing.strategy.geometry;
     if (inputGeometry != null) {

engine/src/flutter/lib/web_ui/test/engine/pointer_binding_test.dart

@@ -2609,6 +2609,88 @@ void testMain() {
     },
   );
 
+  test('ignores pointerId on coalesced events', () {
+    final _MultiPointerEventMixin context = _PointerEventContext();
+    final List<ui.PointerDataPacket> packets = <ui.PointerDataPacket>[];
+    List<ui.PointerData> data;
+    ui.PlatformDispatcher.instance.onPointerDataPacket = (ui.PointerDataPacket packet) {
+      packets.add(packet);
+    };
+
+    context.multiTouchDown(const <_TouchDetails>[
+      _TouchDetails(pointer: 52, clientX: 100, clientY: 101),
+    ]).forEach(rootElement.dispatchEvent);
+    expect(packets.length, 1);
+
+    data = packets.single.data;
+    expect(data, hasLength(2));
+    expect(data[0].change, equals(ui.PointerChange.add));
+    expect(data[0].synthesized, isTrue);
+    expect(data[0].device, equals(52));
+    expect(data[0].physicalX, equals(100 * dpi));
+    expect(data[0].physicalY, equals(101 * dpi));
+
+    expect(data[1].change, equals(ui.PointerChange.down));
+    expect(data[1].device, equals(52));
+    expect(data[1].buttons, equals(1));
+    expect(data[1].physicalX, equals(100 * dpi));
+    expect(data[1].physicalY, equals(101 * dpi));
+    expect(data[1].physicalDeltaX, equals(0));
+    expect(data[1].physicalDeltaY, equals(0));
+    packets.clear();
+
+    // Pointer move with coaleasced events
+    context.multiTouchMove(const <_TouchDetails>[
+      _TouchDetails(pointer: 52, coalescedEvents: <_CoalescedTouchDetails>[
+        _CoalescedTouchDetails(pointer: 0, clientX: 301, clientY: 302),
+        _CoalescedTouchDetails(pointer: 0, clientX: 401, clientY: 402),
+      ]),
+    ]).forEach(rootElement.dispatchEvent);
+    expect(packets.length, 1);
+
+    data = packets.single.data;
+    expect(data, hasLength(2));
+    expect(data[0].change, equals(ui.PointerChange.move));
+    expect(data[0].device, equals(52));
+    expect(data[0].buttons, equals(1));
+    expect(data[0].physicalX, equals(301 * dpi));
+    expect(data[0].physicalY, equals(302 * dpi));
+    expect(data[0].physicalDeltaX, equals(201 * dpi));
+    expect(data[0].physicalDeltaY, equals(201 * dpi));
+
+    expect(data[1].change, equals(ui.PointerChange.move));
+    expect(data[1].device, equals(52));
+    expect(data[1].buttons, equals(1));
+    expect(data[1].physicalX, equals(401 * dpi));
+    expect(data[1].physicalY, equals(402 * dpi));
+    expect(data[1].physicalDeltaX, equals(100 * dpi));
+    expect(data[1].physicalDeltaY, equals(100 * dpi));
+    packets.clear();
+
+    // Pointer up
+    context.multiTouchUp(const <_TouchDetails>[
+      _TouchDetails(pointer: 52, clientX: 401, clientY: 402),
+    ]).forEach(rootElement.dispatchEvent);
+    expect(packets, hasLength(1));
+    expect(packets[0].data, hasLength(2));
+    expect(packets[0].data[0].change, equals(ui.PointerChange.up));
+    expect(packets[0].data[0].device, equals(52));
+    expect(packets[0].data[0].buttons, equals(0));
+    expect(packets[0].data[0].physicalX, equals(401 * dpi));
+    expect(packets[0].data[0].physicalY, equals(402 * dpi));
+    expect(packets[0].data[0].physicalDeltaX, equals(0));
+    expect(packets[0].data[0].physicalDeltaY, equals(0));
+
+    expect(packets[0].data[1].change, equals(ui.PointerChange.remove));
+    expect(packets[0].data[1].device, equals(52));
+    expect(packets[0].data[1].buttons, equals(0));
+    expect(packets[0].data[1].physicalX, equals(401 * dpi));
+    expect(packets[0].data[1].physicalY, equals(402 * dpi));
+    expect(packets[0].data[1].physicalDeltaX, equals(0));
+    expect(packets[0].data[1].physicalDeltaY, equals(0));
+    packets.clear();
+  });
+
   test(
     'correctly parses cancel event',
     () {
@@ -3419,7 +3501,26 @@ mixin _ButtonedEventMixin on _BasicEventContext {
 }
 
 class _TouchDetails {
-  const _TouchDetails({this.pointer, this.clientX, this.clientY});
+  const _TouchDetails({
+    this.pointer,
+    this.clientX,
+    this.clientY,
+    this.coalescedEvents,
+  });
+
+  final int? pointer;
+  final double? clientX;
+  final double? clientY;
+
+  final List<_CoalescedTouchDetails>? coalescedEvents;
+}
+
+class _CoalescedTouchDetails {
+  const _CoalescedTouchDetails({
+    this.pointer,
+    this.clientX,
+    this.clientY,
+  });
 
   final int? pointer;
   final double? clientX;
@@ -3478,6 +3579,10 @@ class _PointerEventContext extends _BasicEventContext
 
   @override
   List<DomEvent> multiTouchDown(List<_TouchDetails> touches) {
+    assert(
+      touches.every((_TouchDetails details) => details.coalescedEvents == null),
+      'Coalesced events are not allowed for pointerdown events.',
+    );
     return touches
         .map((_TouchDetails details) => _downWithFullDetails(
               pointer: details.pointer,
@@ -3541,6 +3646,7 @@ class _PointerEventContext extends _BasicEventContext
               clientX: details.clientX,
               clientY: details.clientY,
               pointerType: 'touch',
+              coalescedEvents: details.coalescedEvents,
             ))
         .toList();
   }
@@ -3570,8 +3676,9 @@ class _PointerEventContext extends _BasicEventContext
     int? buttons,
     int? pointer,
     String? pointerType,
+    List<_CoalescedTouchDetails>? coalescedEvents,
   }) {
-    return createDomPointerEvent('pointermove', <String, dynamic>{
+    final event = createDomPointerEvent('pointermove', <String, dynamic>{
       'bubbles': true,
       'pointerId': pointer,
       'button': button,
@@ -3580,6 +3687,26 @@ class _PointerEventContext extends _BasicEventContext
       'clientY': clientY,
       'pointerType': pointerType,
     });
+
+    if (coalescedEvents != null) {
+      // There's no JS API for setting coalesced events, so we need to
+      // monkey-patch the `getCoalescedEvents` method to return what we want.
+      final coalescedEventJs = coalescedEvents
+          .map((_CoalescedTouchDetails details) => _moveWithFullDetails(
+                pointer: details.pointer,
+                button: button,
+                buttons: buttons,
+                clientX: details.clientX,
+                clientY: details.clientY,
+                pointerType: 'touch',
+              )).toJSAnyDeep;
+
+      js_util.setProperty(event, 'getCoalescedEvents', js_util.allowInterop(() {
+        return coalescedEventJs;
+      }));
+    }
+
+    return event;
   }
 
   @override
@@ -3620,6 +3747,10 @@ class _PointerEventContext extends _BasicEventContext
 
   @override
   List<DomEvent> multiTouchUp(List<_TouchDetails> touches) {
+    assert(
+      touches.every((_TouchDetails details) => details.coalescedEvents == null),
+      'Coalesced events are not allowed for pointerup events.',
+    );
     return touches
         .map((_TouchDetails details) => _upWithFullDetails(
               pointer: details.pointer,
@@ -3670,6 +3801,10 @@ class _PointerEventContext extends _BasicEventContext
 
   @override
   List<DomEvent> multiTouchCancel(List<_TouchDetails> touches) {
+    assert(
+      touches.every((_TouchDetails details) => details.coalescedEvents == null),
+      'Coalesced events are not allowed for pointercancel events.',
+    );
     return touches
         .map((_TouchDetails details) =>
             createDomPointerEvent('pointercancel', <String, dynamic>{