Created attachment 326086 [details]
Sample program that exhibits issue

As specified in the documentation, gdk_window_reparent must reposition the child window with respect to its parent window.

However, the current implementation does not achieve this in certain cases. For example, if Window C is reparented to Window B which is a child of Window A (toplevel), Window C will actually be positioned with respect to Window A.

This is displayed in the attached sample code. Using the current implementation, the green coloured window is incorrectly positioned in the upper-left hand corner of the toplevel window. It should actually be positioned with respect to the red coloured window.

The reason for this is that gdk_window_reparent calls MoveWindow to perform the move. This is in contrast to all other move/resize functions within GDK W32, which uses SetWindowPos.

Another effect of using MoveWindow is that if a window is reparented to be toplevel, it will be sized without accounting for the size of window decorations. This will result in the client area being smaller than expected.

The expected behaviour as described has been confirmed to work under Linux(X11).

A reasonable fix would be to remove the call to MoveWindow and to replace it with a call to one of the pre-existing move/resize functions, which take all of this into account, and furthermore repositions the window with respect to the parent correctly.