http://bugs.winehq.org/show_bug.cgi?id=14312
--- Comment #5 from Reece Dunn msclrhd@gmail.com 2010-02-02 16:16:01 --- I have tested this with another MDI application (multiedit - http://www.programmersheaven.com/download/2555/download.aspx) and this exhibits the same issue.
This does not happen when running multipad on Windows XP (XP correctly draws the background of the client).
In addition to this, adding an InvalidateRect call to the MDI client window after an MDI child has been created (see create_child_window in winefile.c) causes the winefile MDI client window to be redrawn as exxpected, but does not resolve the issue for other applications such as multipad.
I suspect that this is because WM_ERASEBACKGROUND does not correctly repaint the MDI client background.