I am trying to debug (someone else's) site where there is a button that opens a new browser window that leads to a 404. I am trying to determine if the new window is opened with a POST, but the old window's dev network tools don't show the request since it occurs in a new window. And when I open the dev tools in the new window, the request has already been made and doesn't show up.
How do I view this network request in the dev tools?
You can open the Browser Console, beware it's not the same as the Web Console.
From there, you'll be able to monitor all Requests made from all contexts, by switching-on the "Requests" toggle.