That's a sensible thing for it to do, but if you've always wished for a screensaver, your prayers may soon be answered: a recent Chromium commit shows that Chrome OS could pick up a Chromecast ...

Chrome OS displays a list of photo albums synced down from Google Photos for users to select from Tap or click on the album you’d like to showcase when your Chromebook goes into standby mode ...

That may soon change, though, as a commit strongly hints that Chrome OS may be adopting the Chromecast backdrop images for a new screensaver-like “Ambient Mode” for Chromebooks.