1) Install https://chrome.google.com/webstore/detail/google-hangouts/knipolnnllmklapflnccelgolnpehhpl
2) Launch the app and inspect the iframe.
3) Select some element.
4) Click the Event Listeners and select the wheel event handler.
5) The source code lists something like (program):85
6) Clicking (program):85 launches a new tab with the URI of chrome-devtools://devtools/bundled/inspector.html?&remoteBase=https://chrome-devtools-frontend.appspot.com/serve_file/@aa7c8d23c098e96a388ffedf6698230bda650bb3/&dockSide=undocked

I'd expect at #6 it would load the code that is relevant for the event listener and I could set breakpoints on it.