Screen blinks after open html attachment

[Yuliia]

Minimum Steps to Reproduce:
1. load local html resource in WKWebView
F.e.:
        let htmlPath = Bundle.main.path(forResource: "index", ofType: "html")
        let htmlUrl = URL(fileURLWithPath: htmlPath!, isDirectory: false)
        webView.loadFileURL(htmlUrl, allowingReadAccessTo: htmlUrl)
2. Observe html file rendering process

Actual Result:
Content of file displays, screen starts to blink

Expected Result:
Content of file should be displayed without problems

Environment Details :
iPadPro iOS 13.3

[Simon Fraser (smfr)]

Please provide the entire project and the HTML that reproduces this. We need more than just a description of the problem. If you need to provide confidential info, please use bugreport.apple.com.

[Yuliia]

Created attachment 386093 [details]
Sample which may be used to reproduce described behavior

Provided project and the HTML that reproduces described issue.

[Simon Fraser (smfr)]

Blink looks like toggling between two viewport sizes.

[Radar WebKit Bug Importer]

<rdar://problem/58089555>

[Simon Fraser (smfr)]

I can reproduce.

[Wenson Hsieh]

This looks pretty bad.

For the time being, you can work around the issue by adding this line:

…
  let configuration = WKWebViewConfiguration()
+ configuration.defaultWebpagePreferences.preferredContentMode = .mobile

  webView = WKWebView(frame: view.bounds, configuration: configuration)
…