This is good voodoo stuff indeed!

You don't even need an instance of the class, you just need to define it?
That is correct.

What if you don't define the class, but just declare an instance of QWebEngineView in main() but never show() it?
Declaring an instance of QWebEngineView to the main() do work:

QWebEngineView l_QWebEngineView;

Or simply include the QWebEngineView header file in main()?
Good call but nope.