Tentative bugfix: Scrolling step is skipped when element is unregistered (...)

(...) due to TreeWalker.MoveNext() being called twice in a row.
Trello card: https://trello.com/c/pAwL8FEP/84-si-on-defile-sur-radar-et-clavier-et-qu-on-arrete-le-radar-au-moment-ou-le-defilement-est-sur-le-radar-on-arrive-directement-sur