Use non-global actor protocols with global actor types

nonglobalactor.swift

import AppKit

// Make a wrapper for the delegate that maps delegate methods to functions
// (Making it a class/inherit from NSObject as needed)
final class TextLayoutManagerDelegateAdapter: NSObject {
	public typealias TextLayoutFragmentProvider = (_ location: NSTextLocation, _ textElement: NSTextElement) -> NSTextLayoutFragment

	public let textLayoutFragmentProvider: TextLayoutFragmentProvider

	public init(
		textLayoutFragmentProvider: @escaping TextLayoutFragmentProvider
	) {
		self.textLayoutFragmentProvider = textLayoutFragmentProvider
	}
}

// Add conformance to the MainActor-incompatible protocol,
// calling out to those function properties
extension TextLayoutManagerDelegateAdapter: NSTextLayoutManagerDelegate {
	func textLayoutManager(
		_ textLayoutManager: NSTextLayoutManager,
		textLayoutFragmentFor location: NSTextLocation,
		in textElement: NSTextElement
	) -> NSTextLayoutFragment {
		textLayoutFragmentProvider(location, textElement)
	}
}

final class TextViewController: NSViewController {
	// create the adapter in your MainActor type, accessing whatever internal methods you need
	private lazy var delegateAdapter: TextLayoutManagerDelegateAdapter? = {
		.init(textLayoutFragmentProvider: { self.textLayoutFragment(for: $0, in: $1) })
	}()

	init() {
		super.init(nibName: nil, bundle: nil)

		// finally, assign the delegate once self is inited
		textView.textLayoutManager?.delegate = self.delegateAdapter
	}
}