I found I could fix this by deleting the app from the simulator. Still, this is a bug that should be looked at.

In case someone else is looking for a solution (for MacOs), I discovered that although you can call ScrollView(.horizontal) or ScrollView(.vertical), writing ScrollView(.horizontal, .vertical) does not work. For some reason you have to add brackets: ScrollView([.horizontal, .vertical]).

This behaviour is incredibly frustrating. Window resizing is an integral part of MacOS workflow and has been for years. To remove the capability without warning and forcing reworking acres of code is not acceptable. The new default is messy to implement, and the only upside seems to be a more seamless ability to write universal apps. But there should be -- at very least -- an option to stipulate that the app is only for Mac and a return to the old defaults. In my apps there are dozens of popup sheets that (because their content is dynamic) can require resizing. To get them back to how they were before Sequoia, I would need to recode using the very old and clunky NS Window Controller, and for more complex layouts, I might need to use NSViewRepresentable to create custom NSViews and integrate them into the NSWindow. Nightmare.