Detached dock widgets can not be re-attached to main window

After dragging a dock widget from a main window, dragging it back to the dock widgets region no longer allows it to be drop back to the main window.

Build and run dockwidgets example. Drag one of the 2 dock widgets away from the main window. Click and drag the detached dock widget back the its old location in the main window doesn't hilight the area to indicate acceptable drop.

The same example run and behave as expected in 4.8.2.