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Summary:
My app has a WKWebView that displays a web page containing some webGL content. I use drawViewHierarchyInRect to render the WKWebView to an image after the web page has loaded. This works fine on the simulator but on a phone the webGL content is not rendered to the image. The rest of the web page is rendered OK but on the phone the drawViewHierarchyInRect call returns false.

Steps to Reproduce:
I have created a test Xcode project to illustrate the problem:

https://github.com/jedsoane/TestWKWebViewImageCapture

The basic idea is:

1. Added a WKWebView. Get it to load a page containing webGL content, e.g. http://madebyevan.com/webgl-water/
2. Add a UIImageView to preview the image we will create.
3. Add a button to connect to a method that calls drawViewHierarchyInRect e.g.

    @IBAction func takePhoto(sender: AnyObject)
    {
        UIGraphicsBeginImageContextWithOptions(webView.bounds.size, true, 0.0)
        
        let drewOK = webView.drawViewHierarchyInRect(webView.bounds, afterScreenUpdates: true)

        if (!drewOK)
        {
            print("drawViewHierarchyInRect failed")
        }

        let overlayImage = UIGraphicsGetImageFromCurrentImageContext()
        UIGraphicsEndImageContext()

        photoPreview.image = overlayImage
}

Run the app, wait for page to load, tap the button to create an image.

Expected Results:
I would expect the webGL to be rendered to the image along with the rest of the web page content.

Actual Results:
Web page content, excluding the webGL canvas, is rendered to the image.

Version:
iOS 9.3.2 (13F69)

Notes:
Also tested on the iOS 10 beta 2. Same problem there.

Configuration:
iPhone 6S and iPhone 6S Plus