QListWidget: keyboard navigation refuses to work if there are some hidden items at the top.

Populate a QListWidget (List mode) with some items so the vertical scrollbar appears.
If you scroll with PageUp/PageDown/Home/End, all is fine.

Now set several items at the top to hidden (but so that the scrollbar still visible).
Keyboard navigation will stop working afterwards.

Test case main.cpp to reproduce
================================
#include <QtGui>

int main(int argc, char** argv)
{
QApplication app(argc, argv);
QListWidget w;

for( int i = 0; i < 400; i++ )

{ QListWidgetItem* item = new QListWidgetItem( QString::number(i)+" Line", &w ); }

for( int i = 0; i < 20; i++ )
w.setItemHidden(w.item, true);

w.show();
return app.exec();
}