Submitted as FB9835060

Here's what I see when it's not working, and I can reproduce this issue on 3 different 12.1 systems (M1 and Intel):

If I can come up with a reproducible case, I will submit a FB report. I think I'm now seeing the problem with Apple screensavers as well. If anyone can try this test, I'd appreciate it: Test 1: Open System Prefernces / Desktop & Screen Saver Select the "Message" screensaver Click Screen Saver Options and enter some text to display "foobar" Result: the apple logo and text are displayed. Test 2: Close System Prefernces Open System Prefernces / Desktop & Screen Saver Result: although the Message screensaver is selected, the Preview is now blank If you click Screen Saver Options and then Cancel, the Preview now shows up Repeating the test with the "Hello" screensaver does not show the bug.

Interesting - if seems that if we set a 0.5 second timer to call StartAnimation from within the Initialize method, it works again. This is probably safe as long as your screensaver can gracefully deal with StartAnimation getting called more than once.

I've seen this on macOS and it seems to be caused by some videos being rendered in hardware. A solution is to use javascript to create a canvas, draw the currebt video frame into the canvas, then show the canvas while you take the screenshot. That seems to force the video content to really get included in the webView properly and then it shows up in the screenshot.

More info This only seems to affect Mojave (10.14) The bug seems to crop up depending on the order in which you installed Safari 14 and Security Update 2020-005. If you installed Safari 14 after the security update, then everything is fine. see mrmacintosh.com/mojave-2020-005-security-update-causing-major-problems-updated/

Also seen in the very popular Aerial screen saver https://github.com/JohnCoates/Aerial/issues/1107

This has been submitted as FB8745172