I would like to evaluate JavaScript before page's document is constructed and its scripts evaluated, in essence emulating Page.addScriptToEvaluateOnNewDocument of Chrome Devtools Protocol.
I tried to evaluate the JS code on loadStarted and loadProgress events of QWebEngineView but the script was evaluated only after page fully loaded.
You can use the QWebEngineScript to do this. It allows you to set an injection point and execution world for your script (similar to e.g. greasemonkey).