Reset the velocity Kalman filter when the mouse enters a window

If we always tracked the mouse, we could always have accurate velocity;
but most of the time, when the mouse enters a top-level window, we don't
know how fast it was moving at that time, unless the application has
requested a window-system mouse grab.  It's better to assume that any
residual velocity stored in the persistent QEventPoint instance (in
QPointingDevicePrivate::activePoints) is inaccurate, and just start over
from zero.  Especially for the sake of autotest sanity.

Task-number: QTBUG-88346
Change-Id: Id6c4fbffb8a86a8ab50a09f09aa62125d10155b4
Reviewed-by: Fabian Kosmale <fabian.kosmale@qt.io>

src/gui/kernel/qguiapplication.cpp

@@ -2417,7 +2417,19 @@ void QGuiApplicationPrivate::processEnterEvent(QWindowSystemInterfacePrivate::En
 
     currentMouseWindow = e->enter;
 
+    // TODO later: EnterEvent must report _which_ mouse entered the window; for now we assume primaryPointingDevice()
     QEnterEvent event(e->localPos, e->localPos, e->globalPos);
+
+    // Since we don't always track mouse moves that occur outside a window, any residual velocity
+    // stored in the persistent QEventPoint may be inaccurate (especially in fast-moving autotests).
+    // Reset the Kalman filter so that the velocity of the first mouse event after entering the window
+    // will be based on a zero residual velocity (but the result can still be non-zero if the mouse
+    // moves to a different position from where this enter event occurred; tests often do that).
+    const QPointingDevicePrivate *devPriv = QPointingDevicePrivate::get(event.pointingDevice());
+    auto epd = devPriv->queryPointById(event.points().first().id());
+    Q_ASSERT(epd);
+    QMutableEventPoint::from(epd->eventPoint).setVelocity({});
+
     QCoreApplication::sendSpontaneousEvent(e->enter.data(), &event);
 }