gtk_tree_view_motion_resize_column() does this:
  if (event->is_hint || event->window != widget->window)
    gtk_widget_get_pointer (widget, &x, NULL);
  else
    x = event->x;

That's wrong; with is_hint events it should use the event's coordinates and call gdk_event_request_motions() at the end.  (Or for the case where the windows don't match (do they actually not?), it should translate the event's coordinates to be with respect to the appropriate window.)

Also, when the mouse is released to end resizing a column, the code doesn't update the column width with respect to the ButtonRelease event's coordinates.  It should do that instead of assuming that the last motion event provided the final coordinates.