I believe the behavior you are referring to is when you are scrolling a long form layout that contains certain widgets, e.g. combo boxes, with the mouse wheel and when the mouse hovers over those widgets the scroll wheel changes its value instead of continuing to scroll the layout.
QGIS uses the Qt GUI toolkit and this behavior is a 'feature' on some platforms and considered a 'bug' on others. Specially, it is very much non-standard Mac OS X GUI behavior:
If you are creating your own form for a PyQGIS plugin, you can fix this by subclassing the Qt widget and reimplementing its wheelEvent:
Just setting the focus for the widget to something non-wheel does not fix the issue. Installing an event filter on the widget will also work, but may slow down the responsiveness of your GUI.
As far as the widgets in QGIS go, every one of them that exhibits the behavior would need to be subclassed to take care of this Qt-specific issue, which will probably not be happening anytime soon. The issue really needs to be addressed by the Qt project and for particular platforms where it is non-standard behavior.