Under certain circumstances Carla would crash when frequent updates on
the QGraphicsScene occurred, especially in combination with scrolling or
panning by the user. Backtraces showed either a segfault in
QGraphicsItem::parentItem() or around
QGraphicsSceneBspTree::climbTree(). While the first trace points towards
missing calls to pepareGeometryChange() in combination with custom
boundingRect() methods, inserting these did not yield a positive change,
though people with this issue also had success in disabling the
ItemIndexingMethod of their QGraphicsScene[0][1]. Setting this to
NoIndex solves this problem.

The original problem is reproducible with an ever-reconnecting client:

$ while true; do klick 120 & (sleep 0.1; killall klick); done

[0] https://bugreports.qt.io/browse/QTBUG-18021?focusedCommentId=367566&page=com.atlassian.jira.plugin.system.issuetabpanels%3Acomment-tabpanel#comment-367566
[1] https://forum.qt.io/topic/71316/qgraphicsscenefinditembsptreevisitor-visit-crashes-due-to-an-obsolete-paintevent-after-qgraphicsscene-removeitem