Are you using Web Designer 9? This is what you have listed in your TalkGraphics profile.
Web Designer Premium 11 and 12 (365) has a Sticky feature. Sticky can do what you want and make the navigation bar remain in the same place.
I don't think this can be done in Web Designer 9 unless you use an ifixed layer.
You can see an example of Sticky objects here. http://www.collectorvehicleappraisalsofnewmexico.com/ This is not a navigation bar, but it could be. The navigation bar at the top of the page is sticky too.
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