WKWebView Autolayout with magnification

When a magnification is altered from default 1.0 (100%) I'm trying to maintain my orignal layout - set in IB like so:

    func fit(_ childView: NSView, parentView: NSView) {
        childView.translatesAutoresizingMaskIntoConstraints = false
        childView.topAnchor.constraint(equalTo: parentView.topAnchor).isActive = true
        childView.leadingAnchor.constraint(equalTo: parentView.leadingAnchor).isActive = true
        childView.trailingAnchor.constraint(equalTo: parentView.trailingAnchor).isActive = true
        childView.bottomAnchor.constraint(equalTo: parentView.bottomAnchor).isActive = true
    }

where the childView is the webView and its parent container view - fit() called from viewDidLoad(). The result is the view is pegged to top left, nice but I'like the window content to match the now smaller or larger webView - using scroll bars if needbe. Any user attempt to resize window, being to skew the layout until they resume default.

Can someone suggestt how to go about supporting magnification ?