QQuickEventPoint::reset: don't cancel grabs

Because touchpoints can be released in a different order than they
were pressed, and even the QPA plugins don't guarantee that points
in a QTouchEvent will always stay in the same order while they are
still pressed, we cannot assume that the order of points will not
change every time a touch update occurs.  And QQuickPointerTouchEvent
has a vector m_touchPoints in which the current points come first:
if the first point in that vector is released, it will be reset() to
represent a different touchpoint with a different ID.  So it follows
that QQuickEventPoint::reset doesn't have enough information to
decide when a change of pointId means an implicit ungrab; and this
code was just a failsafe anyway (thus the qWarning).
QQuickPointerTouchEvent::reset() is responsible for identifying
which grabs need to be retained in which points from one event to
the next, and also currently warns if a point was pressed while
still being grabbed from previous interactions.

However, a mouse event only has one point, and we rely on
QQuickEventPoint to retain the grabbers.  Efficiency-wise it
doesn't make sense to clear m_exclusiveGrabber and m_passiveGrabbers
and then re-populate them each time the mouse moves.
Since the risk of reordered points is only for touch events,
clearing the grabbers is now done in QQuickEventTouchPoint::reset().

One manifestation of this bug was in multibuttons.qml: sometimes
pressing a second button would cause the first button to be released.
It turned out to occur whenever the touchpoints got reordered.

Change-Id: Ia641f619d763e1a1ea7b59c1e1d08bce9d88e707
Reviewed-by: Jan Arve Sæther <jan-arve.saether@qt.io>

1 files changed, 3 insertions, 7 deletions

diff --git a/src/quick/items/qquickevents.cpp b/src/quick/items/qquickevents.cpp
index 97183c55bc..49ed0f050e 100644
--- a/src/quick/items/qquickevents.cpp
+++ b/src/quick/items/qquickevents.cpp
@@ -534,13 +534,7 @@ QQuickPointerDevice *QQuickPointerDevice::tabletDevice(qint64 id)
 void QQuickEventPoint::reset(Qt::TouchPointState state, const QPointF &scenePos, int pointId, ulong timestamp, const QVector2D &velocity)
 {
     m_scenePos = scenePos;
-    if (m_pointId != pointId) {
-        if (m_exclusiveGrabber) {
-            qWarning() << m_exclusiveGrabber << "failed to ungrab previous point" << m_pointId;
-            cancelExclusiveGrab();
-        }
-        m_pointId = pointId;
-    }
+    m_pointId = pointId;
     m_accept = false;
     m_state = static_cast<QQuickEventPoint::State>(state);
     m_timestamp = timestamp;
@@ -784,6 +778,8 @@ QQuickEventTouchPoint::QQuickEventTouchPoint(QQuickPointerTouchEvent *parent)
 void QQuickEventTouchPoint::reset(const QTouchEvent::TouchPoint &tp, ulong timestamp)
 {
     QQuickEventPoint::reset(tp.state(), tp.scenePos(), tp.id(), timestamp, tp.velocity());
+    m_exclusiveGrabber.clear();
+    m_passiveGrabbers.clear();
     m_rotation = tp.rotation();
     m_pressure = tp.pressure();
     m_ellipseDiameters = tp.ellipseDiameters();