WKWebView takeSnapshotWithConfiguration does not take a snapshot of html5 video (youtube.com) and WebGL (i.e. hardware accelerated)

[Vladymyr]

When the snapshot of WKWebView (not fullscreen mode) is taken with takeSnapshotWithConfiguration on iOS 12.2 (iPhone XR) (and any other supported version) instead of html5 video content (hardware accelerated - any youtube.com video) and any WebGL content - there is a black portion of the screen, other content around is shown normally. I suspect the issues is related to the fact some html5 videos and WebGL content is hardware accelerated and rendered differently.

Exact WebKit version: AppleWebKit/605.1.15

Steps to reproduce:
Take a snapshot of WKWebView (not fullscreen mode) on iOS 12.2 (iPhone XR) with takeSnapshotWithConfiguration of any playing youtube.com video (e.g. https://www.youtube.com/watch?v=CF2BfWYRZoc ) or a page with WebGL content (e.g. https://threejs.org/examples/#webgl_marchingcubes )

Result:
Black part of the screen in the snapshot (image) instead of a content, other web-page content around is visible 

Expected result:
As a user I can take a snapshot of hardware accelerated html5 videos and WebGL content, so they are present on the final snapshot image

Code sample:
  WKSnapshotConfiguration *configuration = [WKSnapshotConfiguration new];
  configuration.rect = CGRectMake(0, 0, boundsWidth, contentHeight);
  configuration.snapshotWidth = @(boundsWidth);
  //webView is WKWebView*
  [webView takeSnapshotWithConfiguration: configuration completionHandler:
      ^(UIImage * _Nullable snapshotImage, NSError * _Nullable error)
      {
        //Here is the code to deal with the image 
      }];

[Radar WebKit Bug Importer]

<rdar://problem/51047322>