[web] Use eventTarget when computing pointer offset (flutter/engine#56949)

These changes are mainly things I missed in flutter/engine#56719

Fixes flutter#159804

Author: mdebbar

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

@@ -18,9 +18,12 @@ import '../window.dart';
 /// 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 `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).
+/// This takes an optional `eventTarget`, because the `event.target` may have
+/// the wrong value for "coalesced" events. See:
+///
+/// - https://github.com/flutter/flutter/issues/155987
+/// - https://github.com/flutter/flutter/issues/159804
+/// - https://g-issues.chromium.org/issues/382473107
 ///
 /// It also takes into account semantics being enabled to fix the case where
 /// offsetX, offsetY == 0 (TalkBack events).
@@ -41,12 +44,12 @@ ui.Offset computeEventOffsetToTarget(
   if (isInput) {
     final EditableTextGeometry? inputGeometry = textEditing.strategy.geometry;
     if (inputGeometry != null) {
-      return _computeOffsetForInputs(event, inputGeometry);
+      return _computeOffsetForInputs(event, eventTarget, inputGeometry);
     }
   }
 
   // On another DOM Element (normally a platform view)
-  final bool isTargetOutsideOfShadowDOM = event.target != actualTarget;
+  final bool isTargetOutsideOfShadowDOM = eventTarget != actualTarget;
   if (isTargetOutsideOfShadowDOM) {
     final DomRect origin = actualTarget.getBoundingClientRect();
     // event.clientX/Y and origin.x/y are relative **to the viewport**.
@@ -70,8 +73,14 @@ ui.Offset computeEventOffsetToTarget(
 /// sent from the framework, which includes information on how to transform the
 /// underlying input element. We transform the `event.offset` points we receive
 /// using the values from the input's transform matrix.
-ui.Offset _computeOffsetForInputs(DomMouseEvent event, EditableTextGeometry inputGeometry) {
-  final DomElement targetElement = event.target! as DomHTMLElement;
+///
+/// See [computeEventOffsetToTarget] for more information about `eventTarget`.
+ui.Offset _computeOffsetForInputs(
+  DomMouseEvent event,
+  DomEventTarget eventTarget,
+  EditableTextGeometry inputGeometry,
+) {
+  final DomElement targetElement = eventTarget as DomElement;
   final DomHTMLElement domElement = textEditing.strategy.activeDomElement;
   assert(targetElement == domElement, 'The targeted input element must be the active input element');
   final Float32List transformValues = inputGeometry.globalTransform;

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

@@ -32,6 +32,7 @@ void doTests() {
   group('computeEventOffsetToTarget', () {
     setUp(() {
       view = EngineFlutterView(EnginePlatformDispatcher.instance, domDocument.body!);
+      EnginePlatformDispatcher.instance.viewManager.registerView(view);
       rootElement = view.dom.rootElement;
       eventSource = createDomElement('div-event-source');
       rootElement.append(eventSource);
@@ -58,6 +59,7 @@ void doTests() {
     });
 
     tearDown(() {
+      EnginePlatformDispatcher.instance.viewManager.unregisterView(view.viewId);
       view.dispose();
     });
 
@@ -101,6 +103,36 @@ void doTests() {
       expect(offset.dy, 110);
     });
 
+    test('eventTarget takes precedence', () async {
+      final input = view.dom.textEditingHost.appendChild(createDomElement('input'));
+
+      textEditing.strategy.enable(
+        InputConfiguration(viewId: view.viewId),
+        onChange: (_, __) {},
+        onAction: (_) {},
+      );
+
+      addTearDown(() {
+        textEditing.strategy.disable();
+      });
+
+      final moveEvent = createDomPointerEvent('pointermove', <String, Object>{
+        'bubbles': true,
+        'clientX': 10,
+        'clientY': 20,
+      });
+
+      expect(
+        () => computeEventOffsetToTarget(moveEvent, view),
+        throwsA(anything),
+      );
+
+      expect(
+        () => computeEventOffsetToTarget(moveEvent, view, eventTarget: input),
+        returnsNormally,
+      );
+    });
+
     test('Event dispatched by TalkBack gets a computed offset', () async {
       // Fill this in to test _computeOffsetForTalkbackEvent
     }, skip: 'To be implemented!');