Arc browser famously had native parts of the UI done in Swift, which admittedly is not a lot: browser tabs, some popups and buttons here and there, a lot of their settings UI is rendered in HTML and is stock Chromium. Which is probably why they actually made a port of it to run on Windows rather than rewriting it into something like Qt or WPF or whatever.

Ladybird browser team planned to start using Swift in their codebase, but it hasn't happen yet.