[REG 5.3->5.4]: Scrolling outside of popup of combo box while popped closes popup

In QTBUG-40656, a fix was applied such that if a QComboBox's popup is showing, and the user scrolls using the scroll wheel while outside of the popup, the popup is closed.

The Gerrit link for this change is: https://codereview.qt-project.org/#/c/93251/4

I don't think that the way the bug was fixed is correct.

On both Macintosh (10.9.5) or Windows 7, the equivalent of a QComboBox in a system dialog does not have this behavior. If you click on the combo box to pop it, move the mouse cursor outside of the popup (such as over some other part of the dialog containing the combo box) and scroll using the scroll wheel, the popup does not close.

On Macintosh, I tested this using System Preferences->General->Appearance. On Windows, I tested using Control Panel->Windows Update->Change Settings->Important Updates.

I can reproduce this using the qtbase/examples/widgets/widgets/styles example experiment.

We have had a few macintosh users report bugs to us that we've tracked down to this change. The users were using trackpads, and instead of simply clicking on an item in the popup, they mostly clicked but also scrolled a tiny bit, and the tiny bit of scroll results in the popup being closed before their click can take effect. Therefore, the user thinks he is clicking on an item, but in fact he is closing the popup. This results in confusion since it's not standard behavior.