A three column NavigationSplitView should give you the design you want. The following article provides an introduction to NavigationSplitView: https://swiftprogramming.com/navigationsplitview-swiftui/

I think you're experiencing a bug in Instruments. Usually when you follow a call, an item gets added to the jump bar. Remove the item from the jump bar by clicking the X button to go back to the root. The jump bar in your screenshot having no items is a sign of a bug.

I recommend installing Xcode 26.5 and see if the update fixes your issue. If you still have the issue, file a bug report.

The Mac App Store might not accept command-line apps. They may require apps to be in a Mac app bundle. One way to build your app for the Mac App Store is to create a simple Mac GUI app project in Xcode and bundle your command-line app with the GUI app. Use the Process class to run the command-line app. You will have to conform to the App Sandbox guidelines to get approved on the Mac App Store. Many Mac apps that bundle command-line apps aren't in the Mac App Store because of the App Sandbox restrictions. You might have to distribute your app from your own website.

What book are you referring to when you say "In the book"? What are you trying to do when you click the Quick Look button? Telling us what you are trying to do can help us tell you what you have to do in Xcode 26 to do the thing you are trying to do.

Doing a search for your error message leads me to believe that the cause of the problem is that Color doesn't conform to Codable. You have to write a transformer for Color that inherits from NSSecureUnarchiveFromDataTransformer. Inheriting from NSSecureUnarchiveFromDataTransformer will make the error/warning go away. Take a look at the following Stack Overflow questions: https://stackoverflow.com/questions/77831799/nskeyedunarchivefromdata-should-not-be-used-to-for-un-archiving-and-will-be-r https://stackoverflow.com/questions/62589985/nskeyedunarchivefromdata-should-not-be-used-to-for-un-archiving-and-will-be-re The second question has an answer that shows how to create a transformer.

The Thread 1: EXC_BAD_ACCESS error shown in your screenshot says you are accessing a nil pointer, causing the app to crash. You are going to have a tough time getting a solution on Apple's forums because not many people on these forums use WinDev Mobile. You have a better chance of getting help at the WinDev Mobile forums. https://forum.pcsoft.fr/en-US/pcsoft.us.windevmobile/page/1.awp

Choose Xcode > Settings to open the settings window. Select Components from the left side of the settings window. Click the Add Platforms button at the bottom of the settings window to open a sheet to select simulators to download.

I saw someone ask this question in a Discord chat room, and it got an answer I'll share here. The inspector view uses a popup button when the inspector view is too narrow. Make the inspector view wider, and you will see the segmented control.

I am not an Apple employee, but there's a good chance most of the Apple apps you mentioned use Mac Catalyst because they were originally written for iOS with UIKit. Porting a UIKit app using Mac Catalyst is going to be faster than writing a Mac version from scratch. I don't see the advantage of using Mac Catalyst for a new SwiftUI app project. SwiftUI supports both iOS and Mac so you can share a lot of the same code and provide a native Mac experience. If you don't want to provide a native Mac experience, you can avoid Mac Catalyst, make an iOS app, and let people with Apple Silicon Macs run the iPad version. I have never used Mac Catalyst so I can't tell you how mature it is.

According to Apple's Mac Catalyst documentation, the purpose of Mac Catalyst is to create a Mac version of an iPad app. Do you have an existing iPad app that you want to port to Mac? If the answer is Yes, using Mac Catalyst will make porting easier than creating a Mac version with AppKit or SwiftUI. However, Apple Silicon Macs can run iPad apps so you can get a Mac version of the app without using Mac Catalyst. If the answer to last paragraph's question is No, you are better off creating a multi-platform SwiftUI app project to make an app that runs on iOS and Mac.

What most likely has happened is Apple made changes to the user interface in Xcode 26 and the tutorial you are following has not been updated for Xcode 26. You can file a request for Apple to update the tutorial. In Xcode choose Help > Provide Feedback to send the request. I have not gone through the tutorial so I can't tell you how to go through the tutorial in Xcode 26. I recommend taking the 100 Days of SwiftUI at Hacking with Swift. https://www.hackingwithswift.com/100/swiftui This course is geared towards beginners. Hacking with Swift also has a dedicated forum for the course (https://www.hackingwithswift.com/forums/100-days-of-swiftui) to ask questions when you run into problems with the course. The forums at Hacking with Swift are more beginner-friendly than Apple's forums.

Start by adding an exception breakpoint in Xcode. An exception breakpoint will pause your app at the point of the crash so you can see where the crash occurs in your code. In Xcode choose Debug > Breakpoints > Creation Exception Breakpoint to set an exception breakpoint. For anyone to provide further help, you have to provide more information. Some helpful information to provide includes the following: When does your app crash, at startup or when something else happens in your app? Is this an iOS or Mac app? Does your app use SwiftUI, UIKit, AppKit, or some other framework? What version of Xcode are you running? Is your app a document app or a regular app? Did you add any code recently that could cause the crash? Or did the app start crashing without you making any code changes? If the exception breakpoint points at code you wrote, provide that code. The stack trace you included doesn't have enough information for anyone to help. People here are going to need to see code to help you.

Some questions. By "SwiftUI instrumentation", do you mean profile your app in Instruments with the SwiftUI instrument? What version of Xcode are you running? What version of iOS is the device running? The new SwiftUI instrument Apple added in Xcode 26 requires iOS 26+. If your device is running an earlier version of iOS, you can't use the new SwiftUI instrument. If you can't use the new SwiftUI instrument in Xcode 26, click the Add Instrument button above the timeline pane graphs and add the legacy View Body and View Properties instruments to profile the app. Also, remove the SwiftUI instrument from the trace.

What did you do after you downloaded the xip file? The xip file is a compressed file. You have to double-click the xip file to expand the compressed file so you can run Xcode.