I solved the problem. I had switched on the "Code Review" button (icon left-pointing arrow above right-point arrow) in the top right area of the middle (code) pane.

Me too! It works in most of my projects, but with my 20-year old macOS project which I converted to a workspace a few years ago, clicking on the project name there never shows that "second sidebar", as you call it. It shows me the raw text of the .pbxproj file instead. I'm not sure, this may be the first time I've tried to edit a Target Setting since updating to Xcode 26 beta. Did you ever figure this out? Thank you.

I'm moving on, Quinn. Thank you for the explanation.

Thank you, Quinn. I attempted to run the store-credentials but got a confused. It asked me for the Path to App Store Connect API private key. I didn't have one of those. So I attempted to create a new key here: https://developer.apple.com/account/resources/authkeys/ To make a long story short, this led me down the wrong keygen rabbit hole. After some more poking around I found this page: https://appstoreconnect.apple.com/access/integrations/api Success. store-credentials accepted the key which I created there, and created a new keychain item which works with notarytool submit. Problem solved. Except to understand why. As you can see from my screenshot, the old keychain item wrapped a password (which was 19 characters), with a name. The new keychain item is much more complex. Its Name is com.apple.gke.notary.tool, Account is le.gke.notary.tool.saved-creds.MyName and its Where is appStoreConnect. Most intriguingly, when I click Show password, and enter the keychain password, nothing happens. So maybe the problem was that the current version of notarytool no longer accepts my old-style simple password keychain item. It now requires this fancy item which based on a private key that Keychain Access can't show me. Is that more or less what happened?

Yes, if I change the swift-argument-parser in the Sparkle project to match that in my project, the problem is solved. But my project does indeed need swift-argument-parser version 2.x. I suppose I could test Sparkle with swift-argument-parser 2.x and submit a pull request to Sparkle if it passes. But I kind of think I should file a bug against Xcode or Swift Package Manager: Should these tools not be able to handle different projects in a workspace requiring different versions of the same remote package? Or is there a better way to fix this of which I am not aware?

Eureka, probably. My workspace includes the Sparkle updater (sparkle-project.org), which is in a different project folder, and depends on swift-argument-parser 0.4.3. Maybe updating Sparkle will fix the problem.

Clean Build Folder does not change the error. Nor does trashing the project's DerivedData folder in Finder.

This bug (no user interface to delete Document Types) is still present in Xcode 14 GM :( If anyone from Apple reads this, please consider just removing the "Info" tab in the Target editor. It seems to be a poorly synced and incomplete user interface for the Info.plist file which often causes more confusion than just editing the Info.plist file manually. (To make that easier, try the app Plist Edit Pro from Fat Cat Software.)

For me, the fix was to stop searching, stop reading, stop thinking, take a deep breath and remember the universal Xcode fix: Product > Clean Build Folder, stupid!! (By the way, this happened immediately after I updated from Xcode 14 Beta to Xcode 14 GM. So it looks like a perennial problem with Xcode updates.)