PSPDFKit ported PDF Viewer (https://pdfviewer.io), a project with over 2 MLOC and lots of UIKit code to the mac via Catalyst. Having played with "Marzipan" in 2018, the team was able to port their huge app in a relative short timeframe. Learn what you have to do after checking the [X] Mac checkbox, how far Catalyst can be pushed, how it interacts with AppKit, and how to work around the worst bugs. We'll show real code, a selection of r̵a̵d̵a̵r̵ Feedback Assistant items, strategies to keep code branching low, and some design alterations that were necessary to make it feel more at home on the Mac.