Add live preview while editing feature

Implemented split-view editor with real-time markdown preview:
- Added Edit/Preview toggle button with toolbar
- Created MarkdownEditor component using NSTextView for raw markdown editing
- Implemented HSplitView with editor on left and live preview on right
- Fixed NSTextView width constraints to prevent unwanted scaling
- Added proper text container configuration for word wrapping
- Text changes update preview in real-time via SwiftUI binding

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

CLAUDE.md

@@ -37,7 +37,7 @@ This repository contains a macOS markdown viewer application built with SwiftUI.
 - [ ] Export to PDF feature
 - [ ] Dark mode support
 - [ ] Custom CSS themes
-- [ ] Live preview while editing
+- [x] Live preview while editing
 - [ ] Search within document
 - [ ] Zoom in/out functionality
 - [ ] Recent files menu

MarkdownViewer/ContentView.swift

@@ -1,4 +1,5 @@
 import SwiftUI
+import AppKit
 
 struct ContentView: View {
     var body: some View {
@@ -24,12 +25,149 @@ struct ContentView: View {
 struct MarkdownDocumentView: View {
     let content: String
     let fileName: String
+    @State private var isEditMode = false
+    @State private var editableContent: String
+    
+    init(content: String, fileName: String) {
+        self.content = content
+        self.fileName = fileName
+        self._editableContent = State(initialValue: content)
+        print("MarkdownDocumentView: Initialized with content length: \(content.count)")
+    }
     
     var body: some View {
-        MarkdownRenderer(content: content)
-            .navigationTitle(fileName)
+        VStack(spacing: 0) {
+            // Toolbar
+            HStack {
+                Button(action: { isEditMode.toggle() }) {
+                    HStack {
+                        Image(systemName: isEditMode ? "eye" : "pencil")
+                        Text(isEditMode ? "Preview" : "Edit")
+                    }
+                }
+                .buttonStyle(.bordered)
+                
+                Spacer()
+                
+                if isEditMode {
+                    Button("Save") {
+                        // TODO: Implement save functionality
+                    }
+                    .buttonStyle(.borderedProminent)
+                }
+            }
+            .padding()
+            .background(Color(NSColor.controlBackgroundColor))
+            
+            // Content area
+            if isEditMode {
+                HSplitView {
+                    // Editor side
+                    VStack(alignment: .leading, spacing: 0) {
+                        Text("Editor")
+                            .font(.headline)
+                            .padding(.horizontal)
+                            .padding(.top, 8)
+                            .padding(.bottom, 4)
+                        
+                        MarkdownEditor(text: $editableContent)
                             .frame(maxWidth: .infinity, maxHeight: .infinity)
                     }
+                    
+                    // Preview side
+                    VStack(alignment: .leading, spacing: 0) {
+                        Text("Preview")
+                            .font(.headline)
+                            .padding(.horizontal)
+                            .padding(.top, 8)
+                            .padding(.bottom, 4)
+                        
+                        MarkdownRenderer(content: editableContent)
+                            .frame(maxWidth: .infinity, maxHeight: .infinity)
+                    }
+                }
+            } else {
+                MarkdownRenderer(content: editableContent)
+                    .frame(maxWidth: .infinity, maxHeight: .infinity)
+            }
+        }
+        .navigationTitle(fileName)
+    }
+}
+
+struct MarkdownEditor: NSViewRepresentable {
+    @Binding var text: String
+    
+    func makeNSView(context: Context) -> NSScrollView {
+        let scrollView = NSScrollView()
+        let textView = NSTextView()
+        
+        // Configure text view
+        textView.delegate = context.coordinator
+        textView.isEditable = true
+        textView.isSelectable = true
+        textView.usesRuler = false
+        textView.isAutomaticQuoteSubstitutionEnabled = false
+        textView.isAutomaticDashSubstitutionEnabled = false
+        textView.isAutomaticTextReplacementEnabled = false
+        textView.font = NSFont.monospacedSystemFont(ofSize: 14, weight: .regular)
+        textView.textColor = NSColor.textColor
+        textView.backgroundColor = NSColor.textBackgroundColor
+        textView.string = text
+        
+        // Properly set up the scroll view hierarchy first
+        scrollView.borderType = .noBorder
+        scrollView.hasVerticalScroller = true
+        scrollView.hasHorizontalScroller = false
+        scrollView.autohidesScrollers = false
+        scrollView.documentView = textView
+        
+        // Configure text container for proper wrapping
+        if let textContainer = textView.textContainer {
+            textContainer.containerSize = NSSize(width: scrollView.contentSize.width, height: CGFloat.greatestFiniteMagnitude)
+            textContainer.widthTracksTextView = true
+        }
+        
+        // Configure text view sizing
+        textView.isVerticallyResizable = true
+        textView.isHorizontallyResizable = false
+        textView.autoresizingMask = [.width]
+        textView.minSize = NSSize(width: 0, height: scrollView.contentSize.height)
+        textView.maxSize = NSSize(width: CGFloat.greatestFiniteMagnitude, height: CGFloat.greatestFiniteMagnitude)
+        
+        print("MarkdownEditor: Created with text length: \(text.count)")
+        
+        return scrollView
+    }
+    
+    func updateNSView(_ scrollView: NSScrollView, context: Context) {
+        guard let textView = scrollView.documentView as? NSTextView else { return }
+        
+        if textView.string != text {
+            print("MarkdownEditor: Updating text from \(textView.string.count) to \(text.count) characters")
+            textView.string = text
+        }
+    }
+    
+    func makeCoordinator() -> Coordinator {
+        Coordinator(self)
+    }
+    
+    class Coordinator: NSObject, NSTextViewDelegate {
+        let parent: MarkdownEditor
+        
+        init(_ parent: MarkdownEditor) {
+            self.parent = parent
+        }
+        
+        func textDidChange(_ notification: Notification) {
+            guard let textView = notification.object as? NSTextView else { return }
+            
+            DispatchQueue.main.async {
+                self.parent.text = textView.string
+            }
+        }
+    }
 }
 
 #Preview {