After a little more "hacking"
For zooms less than 100% this seems to work much better:
//for zooms less than 100 the window gets smaller
if (zoom < 100)
//restore it back to its original size, for some reason divide the zoom level twice!
RadHtmlPlaceholder1.Width /= (zoom / 100) * (zoom / 100);
RadHtmlPlaceholder1.Height /= (zoom / 100) * (zoom / 100);
//the window does not resize for zooms over 100%
//But scrollbars go away
//somehow need to get the scrollbars back
BUT it does not have the correct effect for zooms over 100%
Without the hack, for zooms over 100% the window stays the same size, but the scrollbars are now gone! (Assuming that they are off the visible area of the control.)
With the hack, the window actually gets smaller, and the scrollbars are still gone.
I haven't had a chance to test the hack enough. Specially with other platforms, like Mac. But with the hack in place, I guess I would be happy if I could get the scrollbars back when the zoom is over 100%. (Since the hack is trying to keep the control from resizing, and over 100% it doesn't resize anyway)