Hello,
We've been using the Fullscreen and Daylight widgets in a custom JSAPI 4.27 app. Unfortunately they do not coexist well.
When in full-screen mode (from the Esri widget) the date picker in Daylight fails to open. This is demonstrated here: https://jsfiddle.net/om4vd5en/. When you exit full-screen mode, it functions normally. Behaviour is consistent in Firefox, Chrome and Edge.
If you choose to go full screen using F11, this issue is not present so appears related specifically to the Esri widget.
Kind regards.