When you have hidden items in an QListWidget, the scrollbar and the mouse wheel still act as if no items where hidden.

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 < w.count(); i++ )
w.setItemHidden(w.item, true);

foreach( QListWidgetItem* item, w.findItems("39", Qt::MatchContains))

{ w.setItemHidden(item,false); }

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