Fix resampling of down, up, and remove events. (#67080)

This adds the neccessary synthetic move/hover events to
resampler, which is needed for position to not change as
part of a down, up, or remove event.
Co-authored-by: David Reveman <reveman@google.com>

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

@@ -106,6 +106,18 @@ class PointerEventResampler {
     );
   }
 
+  PointerEvent _toMoveOrHoverEvent(
+    PointerEvent event,
+    Offset position,
+    Offset delta,
+    int pointerIdentifier,
+    Duration timeStamp,
+    bool isDown,
+  ) {
+    return isDown ? _toMoveEvent(event, position, delta, pointerIdentifier, timeStamp)
+                  : _toHoverEvent(event, position, delta, timeStamp);
+  }
+
   Offset _positionAt(Duration sampleTime) {
     // Use `next` position by default.
     double x = _next?.position.dx ?? 0.0;
@@ -224,13 +236,21 @@ class PointerEventResampler {
       // Skip `move` and `hover` events as they are automatically
       // generated when the position has changed.
       if (event is! PointerMoveEvent && event is! PointerHoverEvent) {
+        // Add synthetics `move` or `hover` event if position has changed.
+        // Note: Devices without `hover` events are expected to always have
+        // `add` and `down` events with the same position and this logic will
+        // therefor never produce `hover` events.
+        if (position != _position) {
+          final Offset delta = position - _position;
+          callback(_toMoveOrHoverEvent(event, position, delta, _pointerIdentifier, sampleTime, wasDown));
+          _position = position;
+        }
         callback(event.copyWith(
           position: position,
-          delta: position - _position,
+          delta: Offset.zero,
           pointer: pointerIdentifier,
           timeStamp: sampleTime,
         ));
-        _position = position;
       }
 
       _queuedEvents.removeFirst();
@@ -248,10 +268,7 @@ class PointerEventResampler {
     final PointerEvent? next = _next;
     if (position != _position && next != null) {
       final Offset delta = position - _position;
-      final PointerEvent event = _isDown
-          ? _toMoveEvent(next, position, delta, _pointerIdentifier, sampleTime)
-          : _toHoverEvent(next, position, delta, sampleTime);
-      callback(event);
+      callback(_toMoveOrHoverEvent(next, position, delta, _pointerIdentifier, sampleTime, _isDown));
       _position = position;
     }
   }

packages/flutter/test/gestures/gesture_binding_resample_event_on_widget_test.dart

@@ -97,9 +97,13 @@ void main() {
 
     // Now the system time is epoch + 11ms
     await tester.pump(const Duration(milliseconds: 2));
-    expect(events.length, 3);
+    expect(events.length, 4);
     expect(events[2].timeStamp, currentTestFrameTime() + kSamplingOffset);
-    expect(events[2].runtimeType, equals(PointerUpEvent));
+    expect(events[2].runtimeType, equals(PointerMoveEvent));
     expect(events[2].position, Offset(40.0 / ui.window.devicePixelRatio, 0.0));
+    expect(events[2].delta, Offset(15.0 / ui.window.devicePixelRatio, 0.0));
+    expect(events[3].timeStamp, currentTestFrameTime() + kSamplingOffset);
+    expect(events[3].runtimeType, equals(PointerUpEvent));
+    expect(events[3].position, Offset(40.0 / ui.window.devicePixelRatio, 0.0));
   });
 }