Why is Push not happening inside an iframe?

I have a Vaadin 14 app that relies heavily on Push and works perfectly in normal browser tabs. When the same URL is embedded in an iframe, the Push do not seem to happen. Nothing at all changes on the iframed page. I am using the automatic Push mode.

Is the browser console show some warnings, perhaps some “Not allowed to embed/iframe your vaadin application”?

Check if the different Push transport modes make a difference.