An NSHostingView inline inside an NSTextView

InlineHostTextView.swift

//
//  InlineHostTextView.swift
//  InlineHostTextView
//
//  Created by Stephan Casas on 4/30/23.
//

import SwiftUI;
import AppKit;
import Combine;

// MARK: - Sample Usage / Main Content View
struct ContentView: View {
    var body: some View {
        
        InlineHostTextView(
            "You have {{ content }} new messages.",
            contentWidth: 30,
            contentHeight: 30
        ){
            IncrementingCounter();
        }.font(.boldSystemFont(ofSize: 22))
        
        VStack {
            Button("Increment", action: {countPublisher.send(1)});
            Spacer();
        }
        
    }
}

// MARK: - Sample Counter View
let countPublisher = PassthroughSubject<Int, Never>();
struct IncrementingCounter: View {
    @State var count: Int = 0;
    
    var body: some View {
        ZStack {
            RoundedRectangle(cornerRadius: 12.5)
                .foregroundColor(.purple)
                .shadow(radius: 2.0)
            HStack{
                Spacer()
                Text("\(count)")
                    .foregroundStyle(.white)
                    .font(.system(size: 18, weight: .light))
                    .onReceive(countPublisher) { increment in
                        self.count += increment;
                    }
                Spacer();
            }
        }
    }
}

// MARK: - Custom SwiftUI NSViewRepresentable with NSTextView and NSHostingView
struct InlineHostTextView<Content: View>: NSViewRepresentable {
    
    private var font: NSFont;
    private var color: NSColor;
    
    let displayString: String;
    let contentAnchor: Int;
    
    let contentView: () -> Content;
    let contentSize: NSSize;
    
    init(
        _ withString: String,
        usingFont: NSFont = .systemFont(ofSize: 14, weight: .semibold),
        presentingColor: NSColor = .secondaryLabelColor,
        replacingText: String = "{{ content }}",
        contentWidth: CGFloat,
        contentHeight: CGFloat,
        _ content: @escaping () -> Content
    ) {
        guard let replaceIndex = withString.range(of: replacingText) else {
            fatalError("Replacement text must be present in given string.");
        }
        
        self.contentAnchor = withString.distance(
            from: withString.startIndex,
            to: replaceIndex.lowerBound);
        
        self.displayString = withString.replacingOccurrences(
            of: replacingText,
            with: ""
        );
        
        self.font = usingFont;
        self.color = presentingColor;
        self.contentSize = NSSize(width: contentWidth, height: contentHeight);
        self.contentView = content;
    }
    
    /// Set the color for the display string of this view.
    func color(_ color: NSColor) -> Self {
        var copy = self;
        copy.color = color;
        
        return copy;
    }
    
    /// Set the font for the display string of this view.
    func font(_ font: NSFont) -> Self {
        var copy = self;
        copy.font = font;
        
        return copy;
    }
    
    func makeNSView(context: Context) -> OffsetTextView {
        let textContainer = NSTextContainer(containerSize: NSSize(
            width: CGFloat.greatestFiniteMagnitude,
            height: CGFloat.greatestFiniteMagnitude
        ));
        textContainer.widthTracksTextView = true;
        textContainer.heightTracksTextView = false;
        textContainer.lineFragmentPadding = 0;
        textContainer.maximumNumberOfLines = 1;
        
        let layoutManager = NSLayoutManager();
        layoutManager.addTextContainer(textContainer);
        
        let textStorage = NSTextStorage();
        textStorage.addLayoutManager(layoutManager);
        
        let textView = OffsetTextView();
        
        textView.isEditable = false;
        textView.autoresizingMask = [.width, .height];
        textView.drawsBackground = false;
        textView.allowsUndo = true;
        textView.isAutomaticTextCompletionEnabled = false;
        
        textView.string = self.displayString;
        textView.alignment = .center
        textView.font = self.font;
        textView.textColor = .secondaryLabelColor;
        

        let maxHeight: CGFloat = textView.font!.boundingRectForFont.height;
        textView.offsetY = maxHeight/2;
        
        let attachment = NSTextAttachment();
        attachment.attachmentCell = InlineHostAttachmentCell(
            size: self.contentSize,
            offsetY: maxHeight/4,
            contentView);
        
        textView.textStorage?.insert(
            NSAttributedString(attachment: attachment),
            at: self.contentAnchor);
        
        return textView;
    }
    
    func updateNSView(_ nsView: OffsetTextView, context: Context) { }
    
    // MARK: - NSTextView with Vertical Offset
    class OffsetTextView: NSTextView {
        var offsetY: CGFloat = 0;
        
        override func draw(_ dirtyRect: NSRect) {
            if self.offsetY != 0 {
                textContainerInset = NSSize(width: 0, height: offsetY)
            }
            
            super.draw(dirtyRect)
        }
    }
    
    // MARK: - NSTextAttachmentCell with NSHostingView
    class InlineHostAttachmentCell<Content: View>: NSTextAttachmentCell {
        
        let contentView: () -> Content;
        
        let __size: NSSize;
        let __offsetY: CGFloat;
        
        init(size: NSSize, offsetY: CGFloat, _ content: @escaping () -> Content) {
            self.contentView = content;
            self.__size = size;
            self.__offsetY = offsetY;
            
            super.init();
        }
        
        override func draw(withFrame cellFrame: NSRect, in controlView: NSView?) {
            guard let controlView = controlView else {
                return;
            }
            
            let contentHost = NSHostingView(rootView: contentView());
            
            controlView.addSubview(contentHost);
            contentHost.frame = NSRect(
                x: cellFrame.origin.x,
                y: cellFrame.origin.y + self.__offsetY,
                width: cellFrame.width,
                height: cellFrame.height);
        }
        
        override func cellSize() -> NSSize {
            self.__size;
        }
        
        required init(coder: NSCoder) {
            fatalError("init(coder:) has not been implemented")
        }
    }
}

Did you try adding a second attachment of the same type? In my case, the first one goes blank (still takes up space).