Created attachment 333634 [details]
When a WKWebView is configured with `webView.scrollView.contentInsetAdjustmentBehavior = .never`, DOM elements using `position: fixed` are misplaced. This occurs even though the correct contentInset has been manually applied. When `webView.scrollView.contentInsetAdjustmentBehavior = .automatic` (or not changed), it works correctly. This applies when the web view sits beneath the nav/tab bars. It works correctly if the web view does not underlay nav/tab bars in either configuration.
I've attached a sample project that demonstrates the bug. It is also available on GitHub - https://github.com/zachwaugh/wkwebview-bugs/tree/master/PositionFixedContentInset