Why doesn't Chrome or Firefox show request headers for pending requests?
If you are building a website and put a breakpoint in your server code so that a page cannot be returned until you move past the breakpoint and you (for instance) reload the page in Chrome or Firefox (haven't tested others), you can't see any information about the request.
While debugging, sometimes it's easier to view information about the HTTP request in the browser's dev tools than it is to find that information in the server code. Why am I not able to see HTTP request information until a response is returned by the server?
From: https://bugs.chromium.org/p/chromium/issues/detail?id=294891:
Headers displayed for pending requests are provisional. They represent what request was sent from Blink to Chromium.
We do not update headers until server responds to avoid additional notification used only by DevTools.
To watch real network activity you can use chrome://net-internals
It's not clear what that means, but that's the cited reason.