To reproduce this issue the dock widget should be dragged and dropped out to become a floating window.
To return the focus click anywhere on main window and then on floating dock.
If the dock window is undocked by the float button the floating widget has a focus.

I reproduced this bug with qtbase/widgets/mainwindows/dockwidgets Qt example on Mac.
On Mac 10.8 the dock widget content is missed (see attached screenshots).

There are a lot of similar QTBUGs: 31048, 42818, 44813 (edited: unrelated issues).
Are there any workaround for this issue?