Fix crash in QMacPanGestureRecognizer

The reason it crashed was this:
1. Button was pressed => _panTimer started with the graphics view as
   destination.
2. Button was released => the graphicsview is destroyed
3. 300 ms later: Qt tries to deliver TimerEvent from _panTimer to the
   graphics view. Unfortunately, the graphics view is deleted, but Qt
   doesn't know that... (*crash*)

We therefore chose to start the timer with a destination we are controlling the
lifetime of: the QMacPanGestureRecognizer. Inside the timerEvent of that we can
check if the actual target is already destroyed.

Task-number: QTBUG-60404
Change-Id: Iff8f5b7217de42c4c5cf551ca507f0cff1c99a78
Reviewed-by: Frederik Gladhorn <frederik.gladhorn@qt.io>

1 files changed, 4 insertions, 0 deletions

diff --git a/src/widgets/kernel/qmacgesturerecognizer_p.h b/src/widgets/kernel/qmacgesturerecognizer_p.h
index be3f3266ac..e381a6cc2f 100644
--- a/src/widgets/kernel/qmacgesturerecognizer_p.h
+++ b/src/widgets/kernel/qmacgesturerecognizer_p.h
@@ -55,6 +55,7 @@
 #include "qtimer.h"
 #include "qpoint.h"
 #include "qgesturerecognizer.h"
+#include <QtCore/qpointer.h>
 
 #ifndef QT_NO_GESTURES
 
@@ -88,10 +89,13 @@ public:
     QGesture *create(QObject *target);
     QGestureRecognizer::Result recognize(QGesture *gesture, QObject *watched, QEvent *event);
     void reset(QGesture *gesture);
+protected:
+    void timerEvent(QTimerEvent *ev) override;
 private:
     QPointF _startPos;
     QBasicTimer _panTimer;
     bool _panCanceled;
+    QPointer<QObject> _target;
 };
 
 QT_END_NAMESPACE