This is a CentOS/RedHat bug only and can be reproduced with Qt 4 only and only when running the example via remote connection e.g. ssh X forwarding. It doesn't reproduce with Qt 5.1.1.

The attached example can be used to reproduce the issue.

Expected result:
When moving the scroll bar handle quickly, the handle should stop at the boundaries when either top or bottom is reached.

Actual result:
The scroll bar handle goes over the boundaries when moving quickly from up to down or down to up.