The splash screen widget on a web application is making an iframe embed of the web application not responsive. The web application itself works fine on the arcgis domain. Removing the splash screen widget makes the iframe embed work again. It wasn't an issue until this morning (US Eastern Time Zone). Not sure if it has anything to do with the system maintenance last night?
I tested with various publicly available web applications with the default splash screen widget in Web App Builder - all seem to have the same issue when they are embedded as iframe. Some with a customized splash screen seem to be working fine. See this Test StoryMap with various embed test.