Hello!
I use Safari on macOS, but this likely applies to other browsers on macOS too. My browser is always in fullscreen. When Bitwarden opens a popup window (e.g. when authenticating with a passkey), it opens in a new, separate, fullscreen window. That’s pretty weird, but makes sense considering macOS cannot stack regular windows on top of a fullscreen window.
I don’t know a thing about the web extension APIs used by Bitwarden on Safari/macOS, but I know macOS has a special kind of window for dialog/modal popups that always stay on top, even on top of fullscreen/maximised windows. Would it be possible to use that type of window on macOS, so that the popup window opens on top of the browser, even when the browser is in fullscreen?