Easy to reproduce: Create a new window, add a QTreeWidget with a couple of items then in the application code append a disabled item to the widget:

QTreeWidgetItem *disabledItem = new QTreeWidgetItem(QStringList() << "last");
disabledItem->setDisabled(true);
ui->treeWidget->addTopLevelItem(disabledItem);

Now the page-down and end buttons won’t work on the widget, probably because the last item can’t be selected.

It would make more sense if the last enabled item would be selected instead.