Calculation was working as long as one didn't use per pixel scrolling.
Task-number: QTBUG-48579
Change-Id: Ie02e28b008c5c81ed45d7dd17fed96148c23b598
Reviewed-by: Thorbjørn Lindeijer <bjorn@lindeijer.nl>
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@theqtcompany.com>
Reviewed-by: David Faure <david.faure@kdab.com>