How does Beaker get content to appear on top of the BrowserViews?

I am building an application in Electron that uses BrowserViews. I am exploring Beaker for some inspiration on how to solve an issue I have encountered where I can’t get any content to appear above the BrowserView because it does not live in the dom. beaker does this very well, for example when clicking the settings to the right of the navbar that displays the dropdown. Where can I look in the Beaker source to learn how Beaker does this so well?