Tech Stack Decisions: Unit 1 — Core Editor Engine

Core Dependencies

Technology	Choice	Version	Rationale
Language	Swift 6	6.0+	Modern concurrency, strict safety
UI Framework	AppKit (NSTextView)	macOS 14+	TextKit 2 requires NSTextView; no SwiftUI native rich text editor
Text Engine	TextKit 2	macOS 14+	NSTextContentStorage + NSTextLayoutManager for modern text layout
Markdown Parser	swift-markdown (Apple)	Latest stable	Typed AST, internal cmark-gfm, GFM spec compliance
Syntax Highlighting	Highlightr	Latest stable	highlight.js wrapper for code block coloring
Concurrency	Swift Structured Concurrency	Built-in	async/await, actors for thread-safe parsing
Testing	Swift Testing + XCTest	Built-in	Modern test framework for unit/integration tests

Rejected Alternatives

Technology	Rejected	Reason
WKWebView + ProseMirror	For editor surface	Not native; loses IME, VoiceOver, Undo integration
cmark (direct C binding)	For parser	swift-markdown wraps cmark-gfm and adds typed Swift AST
Ink (John Sundell)	For parser	Less maintained, no GFM tables/task lists
tree-sitter	For incremental parsing	Overkill for Markdown; cmark is fast enough for full reparse
Combine	For reactive pipeline	Unnecessary with @Observable + async/await

TextKit 2 Stack Configuration

NSTextContentStorage (model)
    |
    v
NSTextLayoutManager (layout)
    |
    v
NSTextContainer (geometry)
    |
    v
ShoechooTextView : NSTextView (display)

Key TextKit 2 decisions:

Use NSTextContentStorage (not legacy NSTextStorage) for modern text model
Use NSTextLayoutManager (not legacy NSLayoutManager) for fragment-based layout
NSTextLayoutFragment is the unit of layout invalidation — maps well to our block-level re-rendering
Replace attributed string content per-block via NSTextContentStorage.performEditingTransaction

Build Configuration

Setting	Value
Deployment Target	macOS 14.0 (Sonoma)
Swift Language Version	6
Strict Concurrency	Complete
Optimization (Debug)	-Onone
Optimization (Release)	-O

SPM Dependencies (Package.swift)

dependencies: [
    .package(url: "https://github.com/swiftlang/swift-markdown.git", exact: "0.5.0"),
    .package(url: "https://github.com/raspu/Highlightr.git", exact: "2.2.1"),
]

Note: Exact versions pinned per SECURITY-10. Package.resolved MUST be committed.