[TreeView] pageDown works a bit wrongly.

According to the specification, pageDown should navigate to "the next information pane" but you may see on the attached movie, (starting from approximately half of the movie), that it is not strictly done.

At first, I show, content of treeView. After that, I put focus on ROOT, and press pageDown. selection appears on the "item-1-2", I point on that, and after that press pageDown again. And selection jumps to the "item-3-1". But "item-1-2" is still visible. That is not correct, because such behavior is not "next information pane" or "one information pane down"...

We use strict checking in our check algorithms, so this case should be observed.