The HTML spinner loaded via loadHTMLString was immediately replaced by
the real navigation and never had a chance to render. This is the same
problem Android had with its HTML spinner (though for a different
reason — Android's froze due to main thread saturation).
Use a native UIActivityIndicatorView on a dark overlay, matching the
Android approach with ProgressBar. The spinner is added as a subview
on top of the WKWebView inside a container UIView, and removed in
webView(_:didFinish:) via WKNavigationDelegate.
Also wraps the WKWebView in a container UIView (like Android's
FrameLayout) so the spinner overlay can sit on top independently.
Alex Gleason
59f68efdc7