There does not seem to be a way to accomplish this. The CCS overflow approach has some definitions but none which will make the page to reender as is or reender as is + the number of pixels for the vertical scrollbar. Thus, its either overflow: hidden to hide all scrollbars, overflow-y: auto and overflow-x: hidden to have an auto vertical scrollbar and no horizontal toolbar, for example. But, nothing seems to support that the width of the IFrame should be always the same, with an optional extra spacing for the vertical scrollbar when needed. Because, if you test it, if you define an IFrame, you will see that if you set overflow-y: auto, this will make only the vertical scrollbar appear when needed. However, its spacing is reserved. So, this creates an empty spot. And, as for most users will never need it, we do not want that spacing or visual disabled vertical scrollbar to appear.

There might be another approach to see this. But, this one is quite unique I have to say.