If you need to investigate the proofed or produced DC design source code, its incoming/outgoing network communication, etc., access its Dev Tools (right-click on it and select Show DevTools in the DC proof or press <F12> in the DC proof or external browsers).

Via this tab you can activate a mobile devices emulator, check for JavaScript errors, monitor network activity, etc.

The debugger tool is the same as in Google Chrome because Designer uses Chromium as its embedded web browser.