Scrollbar stays visible too long with "Show scroll bars: When scrolling"

Open System Preferences / General and set scroll bars to show "When scrolling".

1. open the simpletreemodel example
2. expand nodes / change window size, so not all items are visible
3. move the mouse cursor to the right of the window, near the border, where the scroll bar will appear
4. use the mouse wheel to scroll
5. the scroll bar appears

Expected: a short time after stopping to scroll, the scroll bar should disappear (like it behaves for example in Finder and Safari)

Actual: the scroll bar stays visible until the mouse is moved away from the scroll bar

This can be annoying if the scroll bar overlays some interactive control in the view. See QTCREATORBUG-25517 for an example.