With GTK+ 2.20.0, if "GDK_NATIVE_WINDOWS=1" is set in the environment and a program uses a drawing tablet as an extended input device, two problems arise, making the tablet unusable.

First problem is that "ignore_core_events" logic does not work - for each stylus motion event (with pressure axis etc.), a core pointer motion event (with only X and Y, naturally) gets reported to the same window first.
This is because "ignore_core_events" flag in GdkDisplay structure is tested only by proxy_button_event() and proxy_pointer_event(), which do not even get called from _gdk_windowing_got_event() if "_gdk_native_windows" flag is set.

Second problem is that in most circumstances, the coordinates for stylus motion event are reported wrong (offset from those in the paired Core Pointer event). This is because ConfigureNotify from when window manager positions the main window, doesn't result in gdk_event_translate() calling _gdk_input_configure_event() when input window is attached NOT to the main window itself, but to one of its subwindows. And the root coordinates for the input window remain those set initially by gdk_input_set_extension_events().

This breakage resulted from the "Initial version of input support" changeset.
http://git.gnome.org/browse/gtk+/commit/?id=7372379c24d183aa23e33cef031b774a5cb25ce4