[Regression Qt4 to Qt5] QScrollBar usages 5% CPU on idle

Creating a QScrollBar will eat 5% of the CPU when the application is doing absolutely nothing. 0% CPU usage is expected when the application is doing nothing.

The following code will reproduce the issue. The code works as expected in Qt 4.8.1.

#include <QApplication>
#include <QScrollBar>

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);

    QScrollBar* scrollbar = new QScrollBar();
    scrollbar->show();

    return a.exec();
}

If you install an event filter (installEventFilter()) on a QScrollBar, it'll produce a never ending stream of paint events.