QQuickFlickable: fix flicking when item is rotated

QEventPoint::velocity() is in screen coordinates, as documented; it needs
to be transformed to local coordinates in case the Flickable is rotated
and/or scaled. windowToItemTransform() gives us a composite transform
matrix; we remove the translation components to get only the rotation
and scaling.

Also fix the press position calculation in handleReleaseEvent().

With the manual test one can pinch to scale and rotate a Flickable, then
verify that with a single finger, the Flickable gets scrolled by an
appropriate distance in spite of that.

Done-with: Ivan Solovev
Fixes: QTBUG-99639
Change-Id: I60af1dd932835d358baa1422523cc24b2ab046a0
Reviewed-by: Ivan Solovev <ivan.solovev@qt.io>
Reviewed-by: Paul Olav Tvete <paul.tvete@qt.io>
(cherry picked from commit 8068bde891eefe99a43deb83e58166b476f65225)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

2 files changed, 19 insertions, 5 deletions

diff --git a/src/quick/items/qquickflickable.cpp b/src/quick/items/qquickflickable.cpp
index 551c0cf6cf..09f2a87183 100644
--- a/src/quick/items/qquickflickable.cpp
+++ b/src/quick/items/qquickflickable.cpp
@@ -1054,6 +1054,17 @@ void QQuickFlickable::setSynchronousDrag(bool v)
     }
 }
 
+/*! \internal
+    Take the velocity of the first point from the given \a event and transform
+    it to the local coordinate system (taking scale and rotation into account).
+*/
+QVector2D QQuickFlickablePrivate::firstPointLocalVelocity(QPointerEvent *event)
+{
+    QTransform transform = windowToItemTransform();
+    // rotate and scale the velocity vector from scene to local
+    return QVector2D(transform.map(event->point(0).velocity().toPointF()) - transform.map(QPointF()));
+}
+
 qint64 QQuickFlickablePrivate::computeCurrentTime(QInputEvent *event) const
 {
     if (0 != event->timestamp())
@@ -1361,9 +1372,9 @@ void QQuickFlickablePrivate::handleMoveEvent(QPointerEvent *event)
     qint64 currentTimestamp = computeCurrentTime(event);
     const auto &firstPoint = event->points().first();
     const auto &pos = firstPoint.position();
-    QVector2D deltas = QVector2D(pos - q->mapFromGlobal(firstPoint.globalPressPosition()));
+    const QVector2D deltas = QVector2D(pos - q->mapFromGlobal(firstPoint.globalPressPosition()));
+    const QVector2D velocity = firstPointLocalVelocity(event);
     bool overThreshold = false;
-    QVector2D velocity = event->point(0).velocity();
 
     if (q->yflick())
         overThreshold |= QQuickDeliveryAgentPrivate::dragOverThreshold(deltas.y(), Qt::YAxis, firstPoint);
@@ -1395,12 +1406,14 @@ void QQuickFlickablePrivate::handleReleaseEvent(QPointerEvent *event)
 
     bool canBoost = false;
     const auto pos = event->points().first().position();
-    const auto pressPos = event->points().first().pressPosition();
+    const auto pressPos = q->mapFromGlobal(event->points().first().globalPressPosition());
+    const QVector2D eventVelocity = firstPointLocalVelocity(event);
+    qCDebug(lcVel) << event->deviceType() << event->type() << "velocity" << event->points().first().velocity() << "transformed to local" << eventVelocity;
 
     qreal vVelocity = 0;
     if (elapsed < 100 && vData.velocity != 0.) {
         vVelocity = (event->device()->capabilities().testFlag(QInputDevice::Capability::Velocity)
-                ? event->point(0).velocity().y() : vData.velocity);
+                ? eventVelocity.y() : vData.velocity);
     }
     if ((vData.atBeginning && vVelocity > 0.) || (vData.atEnd && vVelocity < 0.)) {
         vVelocity /= 2;
@@ -1415,7 +1428,7 @@ void QQuickFlickablePrivate::handleReleaseEvent(QPointerEvent *event)
     qreal hVelocity = 0;
     if (elapsed < 100 && hData.velocity != 0.) {
         hVelocity = (event->device()->capabilities().testFlag(QInputDevice::Capability::Velocity)
-                     ? event->point(0).velocity().x() : hData.velocity);
+                     ? eventVelocity.x() : hData.velocity);
     }
     if ((hData.atBeginning && hVelocity > 0.) || (hData.atEnd && hVelocity < 0.)) {
         hVelocity /= 2;
diff --git a/src/quick/items/qquickflickable_p_p.h b/src/quick/items/qquickflickable_p_p.h
index 35f221b3cc..fad63c1ef1 100644
--- a/src/quick/items/qquickflickable_p_p.h
+++ b/src/quick/items/qquickflickable_p_p.h
@@ -268,6 +268,7 @@ public:
               const QVector2D &deltas, bool overThreshold, bool momentum,
               bool velocitySensitiveOverBounds, const QVector2D &velocity);
 
+    QVector2D firstPointLocalVelocity(QPointerEvent *event);
     qint64 computeCurrentTime(QInputEvent *event) const;
     qreal devicePixelRatio() const;