Thank you so much. I will read study those links more today and later as I have time. I added a reply with some further info; I wasn't sure if a comment like this would accept source code snippets.

Yes, I've restarted the phone and Xcode. The phone is showing in the Devices window. Other apps compile and run on the phone.

Okay, I got it working now. Strangely, your example (translated to Obj-C) did not work for me until I removed the NSTrackingCursorUpdate flag from the tracking area. If I have that flag, then I have to set the cursor in the cursorUpdate method. My calls to set or push an NSCursor in the mouseEntered/Exited are ignored. If I remove that flag, then the calls in mouseEntered/Exited work as you have them here. Thanks!

(I'm editing this comment after you added some content to your post.) I see now, you're using the mouseEnter/Exit flags with the tracking area. I'll try that now.

Actually I take that back. Even if I can observe option key state, if I change state when the menu is open, the menu just closes, instead of rebuilding and displaying, which would give the AppKit-like behavior.

I put the code that I wrote before in a package. See my answer.

No. I ended up writing my own outline view from scratch, that dealt with lazy disclosure. Maybe I should open source it? I don't know if there is a better way. If you find one, please share it here.

I'd like to do this in SwiftUI and macOS. I can change menus in response to state, but I need a nice way to globally observe the option key.

Okay, thanks! I'll need time to try out these suggestions and post back later if I'm stuck. I wonder how recursive are the security-scoped bookmarks? Could I simply ask for access to /Users/joe or /Users/joe/Workspace and have read/write access to everything under it? I'll find out when I experiment some today.

Yes, it's sandboxed. Should I just turn that off? It was the default when Xcode created the project. It seems like the ideal is to leave it on and enable access to resources as needed. ?

Oops, forgot your last question. The Signing Certificate popup says "Development" now. I'm sorry, I don't know what you mean by "stable code signing identity". For some of my other projects, which are iOS apps deployed in the App Store, that item is not a popup and it does not say "Development". It will say something like "Apple Development: [My Name] ([alpha-numeric code])". This macOS app that I'm asking about has no App Store record (maybe someday).

Okay, "Files and Folders" might work. The first file that gave the error had a path like "~/Workspace/MyProject/main.jl". That "Workspace" directory is where I have a lot of software dev and other projects. And yes, it's a GUI app that the user would normally click on in Finder. I think I want permissions similar to Xcode. The app is going to be editing different kinds of files: scripts, programming language code files, and custom "project" files that are SQLite databases. It's an app for creating 2D and 3G generative art. So it will probably open "Projects" that are directories full of files, but I would also like it to be able to edit random files. (So, similar to how Xcode works in that respect.)

For some of my dead links, I have enough notes about what was in them to find them again using search engines. But for others, no. For this one the note is basically: "remember this about keychain: link". If they were going to break old links, they should have added a way to find by old ID. Eg, a search query like old_id:281700.

Same here, also with a 4000 line Swift file. Now I need to redesign my application code around Xcode's problems.

I just got this working. Thank you! Besides waitUntilCompleted as you recommended, I also switched from MTKView to a lower level technique using CAMetalLayer (following an Apple sample project named "CustomMetalView"). That may not be necessary but in my situation it helped. Now when I'm in LLDB and I step across a change in the data model, I can call a certain Swift function (p debugRender()) and I see the change on my device screen. Sweet!