ScrollBar: Fix custom scrolling behavior
In order to add our custom mouse event handling behavior to the QQuickScrollBar, the only way in Qt 6 is to put the MouseArea in front of it, i.e. with a higher stacking z index, and only handle a subset of events which we are going to fully process ourselves, while rejecting the rest to let them propagate further down to the scroll bar itself.
Edited by ivan tkachenko