Floating QDockWidget which has a vertical title bar when docked can't be docked by dragging on Mac OS X.

1. Build and run attached .cpp. This consists of a main window with two QDockWidget's, both of which have vertical title bars.
2. Drag or double-click one of the two panes so that it's a floating window.
3. The floating window cannot be docked back onto the main window by dragging. Furthermore, there is no indication in the main window that a drop can be performed. Double-clicking the floating window does dock however.
4. If the code is changed so that one pane has a vertical title bar and the other pane has a horizontal title bar, the bug occurs when the floating window is the pane with the vertical title bar but not when it's the pane with the horizontal title bar.
5. All of the above works properly on Win7 and Linux.