Since I’ve had to go re-watch WWDC20’s “Adopt the New Look of macOS” session (at around the 07:03 mark), I thought I’d note it down here – for anyone else late to the update-for-macOS-Big-Sur-game 😉

What we want to achieve is go from this UI (basically standard on any macOS before Big Sur):

to this UI (new UI for Preferences windows on macOS Big Sur):

And it’s fairly easy to do so. All you need to call is (pardon the Objective-C, Yoink for Mac (and iPad and iPhone, for that matter), is still 100% Swift-free..)

self.window.toolbarStyle = NSWindowToolbarStylePreference;

and update your toolbar items’ icons, and you’re all set.

– Matthias
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