If we get a 'down' event, add that device to the active devices. (#13182)

This prevents us from synthesizing an 'add' event if we get a 'move'
event after a 'down' event. This was causing errors because on the
framework side, it will synthesize an 'add' event if it receives a
'down' event, and if you send another 'add' event (e.g. with a 'move'
after a 'down') then it will throw an AssertionError since it already
has received an 'add' event for that pointer.

Fixes https://github.com/flutter/flutter/issues/40385

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

@@ -14,6 +14,7 @@ class PointerBinding {
   /// The singleton instance of this object.
   static PointerBinding get instance => _instance;
   static PointerBinding _instance;
+
   // Set of pointerIds that are added before routing hover and mouse wheel
   // events.
   //
@@ -61,6 +62,7 @@ class PointerBinding {
     newDetector ??= const PointerSupportDetector();
     // When changing the detector, we need to swap the adapter.
     if (newDetector != _detector) {
+      _activePointerIds.clear();
       _detector = newDetector;
       _adapter?.clearListeners();
       _adapter = _createAdapter();
@@ -213,6 +215,8 @@ class PointerAdapter extends BaseAdapter {
     _addEventListener('pointerdown', (html.Event event) {
       final int pointerButton = _pointerButtonFromHtmlEvent(event);
       final int device = _deviceFromHtmlEvent(event);
+      // The pointerdown event will cause an 'add' event on the framework side.
+      PointerBinding._instance._activePointerIds.add(device);
       if (_isButtonDown(device, pointerButton)) {
         // TODO(flutter_web): Remove this temporary fix for right click
         // on web platform once context guesture is implemented.
@@ -452,6 +456,10 @@ class MouseAdapter extends BaseAdapter {
     html.MouseEvent event,
   ) {
     final List<ui.PointerData> data = <ui.PointerData>[];
+    // The mousedown event will cause an 'add' event on the framework side.
+    if (event.type == 'mousedown') {
+      PointerBinding._instance._activePointerIds.add(_mouseDeviceId);
+    }
     if (event.type == 'mousemove') {
       _ensureMouseDeviceAdded(data, event.client.x, event.client.y,
           event.buttons, event.timeStamp, _mouseDeviceId);

lib/web_ui/test/engine/pointer_binding_test.dart

@@ -83,6 +83,41 @@ void main() {
       expect(packets[1].data[0].change, equals(ui.PointerChange.down));
       expect(packets[1].data[0].device, equals(2));
     });
+
+    test('creates an add event if the first pointer activity is a hover', () {
+      List<ui.PointerDataPacket> packets = <ui.PointerDataPacket>[];
+      ui.window.onPointerDataPacket = (ui.PointerDataPacket packet) {
+        packets.add(packet);
+      };
+
+      glassPane.dispatchEvent(html.PointerEvent('pointermove', {
+        'pointerId': 1,
+        'button': 1,
+      }));
+
+      expect(packets, hasLength(1));
+      expect(packets.single.data, hasLength(2));
+
+      expect(packets.single.data[0].change, equals(ui.PointerChange.add));
+      expect(packets.single.data[1].change, equals(ui.PointerChange.hover));
+    });
+
+    test('does not create an add event if got a pointerdown', () {
+      List<ui.PointerDataPacket> packets = <ui.PointerDataPacket>[];
+      ui.window.onPointerDataPacket = (ui.PointerDataPacket packet) {
+        packets.add(packet);
+      };
+
+      glassPane.dispatchEvent(html.PointerEvent('pointerdown', {
+        'pointerId': 1,
+        'button': 1,
+      }));
+
+      expect(packets, hasLength(1));
+      expect(packets.single.data, hasLength(1));
+
+      expect(packets.single.data[0].change, equals(ui.PointerChange.down));
+    });
   });
 }