http://bugs.winehq.org/show_bug.cgi?id=35456
--- Comment #2 from Ken Thomases ken@codeweavers.com --- Yes, I believe this is due to limitations in the Mac driver. In general, the Mac windowing system does not allow one process to draw into the windows of another process. And, unlike X11, there's no real concept of child windows.
From the point of view of the window server, all windows are top-level windows.
Within a process, the frameworks subpartition the window area – for example, Cocoa maintains a view hierarchy within a window – but that has no externally-accessible representation.
You can try using the X11 driver to confirm. Set this in the registry:
[HKEY_CURRENT_USER\Software\Wine\Drivers] "Graphics"="x11,mac"
If I'm correct, a solution would require some mechanism to share window surfaces across processes. That would also be necessary to support virtual desktop mode with the Mac driver, for the same reason.