The level of opacity for the window.
The valid range of opacity is from 1.0 (completely opaque) to 0.0 (completely transparent).
By default the value of this property is 1.0.
This feature is available on Embedded Linux, Mac OS X, X11 platforms that support the Composite extension, and Windows 2000 and later.
This feature is not available on Windows CE.
Note that under X11 you need to have a composite manager running, and the X11 specific _NET_WM_WINDOW_OPACITY atom needs to be supported by the window manager you are using.
Referenced by QComboBox::changeEvent().