What does GIMP do? rectangular selection box implementation question

[v4r4n <console cowboy gmail com>]

I'm trying to make my own 'rectangular selection box' for a plot/graphing
application.

It is suggested that we always use GDK_POINTER_MOTION_HINT_MASK in the
tutorial (http://www.gtk.org/tutorial/x2433.html) and call
gdk_window_get_pointer() to explicitly ask for the position of the pointer.
Does that mean I have to use a separate timer to call *_get_pointer to
update the box on the drawable instead of using the user's mouse movements
while they move with a button pressed?

Well, is this what GIMP does or are there specific situations where GIMP
just uses the normal GDK_POINTER_MOTION_MASK to draw a box from where the
user last clicked to where the mouse currently sits?

Any hints as to where this would be in the GIMP source are appreciated.

~Thanks