Hey! I’m one of the creators of Beaker. Beaker is built with Electron, a wrapper around Chromium, and it doesn’t include any UI elements like tabs or the URL bar. We made those ourselves, and have taken inspiration from a number of browsers.
What you see here is actually the “old” version of Beaker - a lot has changed in the upcoming release. Here are a few tweets with screenshots if you want to see a preview: