Two different behaviors with or without debugging

[kowalsky]

start :-
    new(Frame, frame('Test')),
    send(Frame, open),
    send(Frame, append, new(Pic, picture)),
    send(Pic, height, 600),
    send(Pic, display, new(_Ref1, text('This will disappear in 3 seconds', center, font(screen, roman, 16)))),
    sleep(3),
    send(Pic, clear),
    sleep(3),
    send(Pic, display, new(_Ref2, text('Good bye!', center, font(screen, roman, 16)))),
    sleep(2),
    send(Pic, clear).

The following code behaves in two different ways when running it with and without debugging.
Without debugging: shows an initial text 'This will disappear in 3 seconds' for 8 seconds and then it disappears.
With debugging (gtrace): shows the initial text for 3 sec, then it disappears for 3 secs, then a new text 'Good bye!' appears that disappears in 2 sec.
Why there are two different behaviors of the same code? 

I am using 6.6.6.swi prolog 64 bit version for windows (on windows 8.1).

The same question at stackoverflow.com (without an answer yet):
http://stackoverflow.com/questions/23985197/two-different-behaviors-of-the-same-prolog-code-with-and-without-debugging

[kowalsky]

I checked one more time and I found out the following:
The muse has to be hovering over the form that I see the changes on the canvas.
Note that hovering (moving over the form)!
Just being over the canvas is not sufficient.
-
Is this should be as it is?