Created attachment 18717 [details]
notebook example with some buttons in tabs.

I tested also with the packages from 

http://www.gimp.org/~tml/gimp/win32/downloads.html

with the same results.

(Understanding the following comments require knowledge of the 
gtknotebook.c source code and gdk/win32 workings.)

This seem to be beause the notebook's "event_window" and the 
tab's "tab_label" window are siblings, and the tab_label window is 
below the event_window in the Z-order. Both the event_window and 
tab_label window are GDK_INPUT_ONLY, which means their extended style 
has WS_EX_TRANSPARENT. The windows are shown in the order tab_label 
first, then event_window.

show_window_internal in gdkwindow-win32.c has this interesting code 
snippet:

  if (GetWindowLong (GDK_WINDOW_HWND (window), GWL_EXSTYLE) & 
WS_EX_TRANSPARENT)
    {
      /* Don't really know if this makes sense, can't remember whether
       * this case is handled like this because it is necessary, or
       * if this is just old crap.
       */
      SetWindowPos(GDK_WINDOW_HWND (window), HWND_TOP, 0, 0, 0, 0,
		   SWP_SHOWWINDOW | SWP_NOREDRAW | SWP_NOMOVE | 
SWP_NOSIZE);
      return;
    }

So it seems that this code snippet will set the event_window's Z-
order position to be in front of tab_label's. Will check what happens 
if that code is removed.

Yes indeed, ifdeffing out that code helps. Now, the interesting 
question is, will this break something else... or was the code 
totally unnecessary. Will try to see if I can notice any change in 
the behaviour of testgtk, gtk-demo and GIMP.

*** Bug 121851 has been marked as a duplicate of this bug. ***

The dubious code removed from HEAD and gtk-2-2.