If tabbed view and horizontal scrollbar policy is set in MDI application and multiple windows are created It leaves painting artifacts
To reproduce it add following line of code in mdi example.

MainWindow::MainWindow()
{
mdiArea->setHorizontalScrollBarPolicy(Qt::ScrollBarAsNeeded);
mdiArea->setVerticalScrollBarPolicy(Qt::ScrollBarAsNeeded);
mdiArea->setViewMode(QMdiArea::TabbedView);
....
}

Steps to reproduce:
1. run the app
2. select "file", "new" repeatedly until the scroll (arrows) indicators
appear
3. Observe the white blob at the far left i.e. at first tab.

This is reproducible in windows and osx