Template canvas, control size of template

I am trying to control the width of the templates inside a template canvas
during runtime the template width is affected the width of the scrollbar
in designer the template width is not affected by the scrollbar

When the scrollbar show and hide during runtime, the tmplates in the repeater grow and shrink.
-Is there a way of avoiding this?
-I am trying to show a header above the repeater, but the content of the repeater gets out of alignment during runtime depending on the scrollbar presense