SF GUI hangs

Earlier today I wrote this short didactic algorithm to fulfill a request
on comp.lang.forth:

: ascends ( n1 n2 -- lesser greater )
\ ascends arranges the top two numbers on the stack in ascending order.
           2DUP > IF SWAP THEN ;

: GCD ( u1 u2 -- u ) \ GCD computes the greatest divisor of n1 and n2.
      BEGIN ascends OVER MOD DUP 0= UNTIL DROP ;

"-3 100 GCD" executes, while "3 -100 GCD" hangs. The point is not that
good code exposes a bug in Forth, but that bad (unsafe) code crashes the
window, blocking <alt> and the red "break" icon. Any thoughts?

Jerry

--
      "The rights of the best of men are secured only as the
       rights of the vilest and most abhorrent are protected."
           - Chief Justice Charles Evans Hughes, 1927
     "The only thing necessary for the triumph of evil is for
      good men to do nothing." - attributed to Edmund Burke 1770?
"They that give up essential liberty to obtain a little temporary
safety deserve neither liberty nor safety."  - Benjamin Franklin, 1759
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