[ltk-user] Cannot access mouse coords relative to CANVAS widget

[Jim W]

I am working with SBCL for Linux on an AMD64 machine.
Function ANIMTEST instantiates an LTK window with a CANVAS widget. Two items, 
BARRIER and FOLLOWER, live in the canvas. Both spin continuously, with BARRIER 
at the center of the canvas and FOLLOWER intended to follow the mouse, which is 
not working as intended. My first attempt (see comment) resulted in absolute 
screen coordinates of the mouse being interpreted as relative coordinates within 
the canvas with no account of the offset between the two. After searching 
through ltk.lisp and docs, I found SCREEN-MOUSE-X/Y (Second Attempt, see 
comment). I feel like I am using SCREEN-MOUSE-X & -Y according to the 
documentation, but why is it not working? = Note = The file that contains 
ANIMTEST and the packages that support it load and run with no errors. Functions 
I have defined (UCTK-BEAM, etc.) are tested and run fine.

(defun animtest ()
      "Test a spinning figure in LTK"
      (with-ltk ()
        (let* ((cnvs (make-instance 'canvas :width 400 :height 400))
           (barrier (uctk-beam 200 200 40 20))
           (follower (uctk-beam 0 40 40 20))
           (slp-time 50) ; in ms
           (bar-theta 0)
           (fol-theta 0))
          (labels ((update ()
             (draw barrier nil)
             (draw follower nil)
             (incf bar-theta (/ pi 15))
             (incf fol-theta (/ pi 15))
             (geo:set-theta barrier bar-theta)
             (geo:set-theta follower fol-theta)
             (geo:set-center follower 
                     ;== FIRST ATTEMPT ==
                     (cons (screen-mouse-x cnvs)
                       (screen-mouse-y cnvs)))
                     ; == SECOND ATTEMPT ==
                     ;(cons (canvasx cnvs (screen-mouse-x cnvs))
                    ;   (canvasy cnvs (screen-mouse-y cnvs))))
             (after slp-time #'update)))
        (pack cnvs :fill :both :expand 1)
        (update)))))
Thanks in advance!