When WKWebView enters full screen (i.e. during video playback) it moves itself over to new window and puts another view in its position ("click to exit FullScreen). Currently platform view container on macOS always reparents the webview back essentially breaking full screen.

One option is to let plugin handle this by using an intermediate view. However the platform view infrastructure on macOS already has an intermediate view in place that holds nothing else then the platform view. This makes it fairly trivial to handle this situation in the embedder making it easier for plugin authors.