As a best-selling author, I invite you to explore my books on Amazon . Don't forget to follow me on Medium and show your support. Thank you! Your support means the world! I remember the first time I tried to build something that lived inside Chrome DevTools. I had this weird idea—a panel that would show me every time a variable changed in my React app. I knew content scripts could touch the page, but I wanted the panel to feel native, like the Elements tab or the Console. So I started digging into the extension APIs, and I found out that the browser opens a whole set of tools just for people like us, who want to make custom debugging gadgets. Let me show you what I learned. The whole thing begins with a plain JSON file. Your extension needs a manifest, and inside it you declare a "devtools_page". This is just an HTML file that will be loaded when the user opens DevTools. But that page’s real job is to register your custom panels.…