Transient scrollbars do not work when using style sheets

Background: Mac has a feature called transient scrollbars; in System Preferences/General there is an option for "Show Scrollbars". When not set to always show scrollbars; OSX will only show scrollbars when scrolling (they appear, move, then disappear). Transient scroll bars are composited on top of the widget, to the side or bottom depending on horizontal or vertical. This means the widget is not resized to make space for the scrollbar.

Problem: If a stylesheet is used for QScrollBar then scrollbars are always drawn on top of the widget and do not disappear if scrolling finished. This blocks access to the widget underneath (cannot see or interact).

Expected Behaviour: For transient scrollbars, Qt should only draw the scrollbar when scrolling and composite on top of the widget. When scrolling ends, the scrollbar should not be hidden. Ideally QScrollBar::add-line and ::sub-line styles should also support alpha channel in the background property.