Sledger, I greatly appreciate your and also Drwyd's engagement in explaining that matter and in suggesting solutions!
I've understood that it's caused by shortcomings of Browsers when rendering fixed width-pages - seemingly
regardless of source.

As Website Professional or Xara-Engineer one therefore can read Rik's request as somewhat naive and simply close the case.
But this ignores the fact that some Wysiswyg-Editors do not show the described problem. Never.

While Xara allows for both great Ease and great Freedom in Design of Websites the described page-shifts
and perceived jumping is definedly a painfull side-effect. I do not see this error happen in other Editors such as Adobe Muse or Artisteer
(in html-website-output-mode). Both while clearly being Wysiwyg lso allow creating dynamic width-pages.
I as a Layman can only assume that this somehow has to do with non-pixel-perfect placement of items on Xara's side and maybe Antialiasing
but this may be total rubbish. All I can say is: The mentioned programs clearly allow for less freedom but the nasty effect here does not appear
and edges also tend to render crisper.

To me it seems evident that page-shifts are not very much in Focus of Xara-Development/Management.
Otherwisely they wouldn't have released a Sales-Website for Website-Creation-Programs which exposes quite a prominent visual defect.