We currently pull the live layer tree into the WKThumbnailView even in snapshot mode, before the snapshot arrives, to avoid flashing when transitioning a live WKView to a thumbnail view. This is, however, not useful for unparented WKViews, because their layer tree will be detached and we'll flash anyway. So, only do that when the WKView is parented.
Also, add a white background so if we don't have a snapshot yet, the thumbnail view isn't transparent.
Created attachment 233461 [details]