When the PageDown is pressed, the current's rect cannot be
used to match the item, because we don't know the size of
the rect. Move the rect by the height of the viewport, and
then move the rect upwards until it matches the
button <= viewport's bottom of the item
Fixes: QTBUG-92583
Pick-to: 6.1
Change-Id: I210edc0e8b942984f3fc20e7752c6e1315152ea1
Reviewed-by: Volker Hilsheimer <volker.hilsheimer@qt.io>
c77840d734