As long as sites can get zoom levels, it seems to me like you’ll have to choose one or the other.
Now, if I’m not mistaken, the only way to do that, is via JavaScript, so disabling JS on a site should block that. Alternatively, block or sanitize the JS call used to calculate the canvas vs. window size (maybe via Tampermonkey?). Or just return a window size equal to the canvas size (I’m kind of surprised RFP doesn’t do that).