Add streaming-into-view display pacing

Text chunks are queued and released at 30-80ms intervals for natural
reading pace, matching the spirit of the lightward.com JS client's
MessageStreamController (adapted timing for iOS's smaller chunks).

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: isaacbowen

Lightward/Chat/ChatView.swift

@@ -47,7 +47,7 @@ struct ChatView: View {
                     .frame(maxWidth: 500, alignment: .leading)
                 }
                 .scrollDismissesKeyboard(.interactively)
-                .onChange(of: vm.streamingText) {
+                .onChange(of: vm.displayedText) {
                     withAnimation(.spring(duration: 0.3)) {
                         proxy.scrollTo("bottom", anchor: .bottom)
                     }

Lightward/Chat/ChatViewModel.swift

@@ -8,14 +8,19 @@ final class ChatViewModel {
     var messages: [ChatMessage] = []
     var inputText = ""
     var streaming = false
-    var streamingText = ""
+    var displayedText = ""
     var error: String?
 
     private let store: Store
     private let phoropterTrail: [String]
     private var hasInitiated = false
     private var currentTask: Task<Void, Never>?
 
+    // Streaming display — queue chunks, release at human reading pace
+    private var chunkQueue: [String] = []
+    private var fullText = ""
+    private var displayTask: Task<Void, Never>?
+
     init(store: Store, phoropterTrail: [String]) {
         self.store = store
         self.phoropterTrail = phoropterTrail
@@ -26,7 +31,6 @@ final class ChatViewModel {
     }
 
     /// Initiates the conversation with the phoropter trajectory as context.
-    /// Called from ChatView.onAppear so it fires when the view is actually visible.
     func initiateIfNeeded() {
         guard !hasInitiated else { return }
         hasInitiated = true
@@ -62,8 +66,11 @@ final class ChatViewModel {
 
     private func streamResponse(chatLog: [[String: Any]]) {
         currentTask?.cancel()
+        displayTask?.cancel()
         streaming = true
-        streamingText = ""
+        fullText = ""
+        displayedText = ""
+        chunkQueue = []
         error = nil
 
         currentTask = Task {
@@ -75,13 +82,8 @@ final class ChatViewModel {
                     switch event {
                     case .text(let chunk):
                         chunkCount += 1
-                        streamingText += chunk
-                        // Update or create the assistant message
-                        if let last = messages.indices.last, messages[last].role == .assistant {
-                            messages[last].text = streamingText
-                        } else {
-                            messages.append(ChatMessage(role: .assistant, text: streamingText))
-                        }
+                        fullText += chunk
+                        enqueueChunk(chunk)
 
                     case .started:
                         Log.chat.debug("Stream: started")
@@ -91,22 +93,21 @@ final class ChatViewModel {
                     }
                 }
 
-                streaming = false
-                Log.chat.info("Stream: complete, \(chunkCount) chunks, final length: \(self.streamingText.count)")
+                Log.chat.info("Stream: complete, \(chunkCount) chunks, length: \(self.fullText.count)")
 
                 if chunkCount == 0 {
-                    Log.chat.error("Stream: completed with zero chunks — no content received")
+                    Log.chat.error("Stream: zero chunks received")
+                    streaming = false
                     self.error = "No response received"
                     ErrorReporter.report(category: "chat", message: "Stream completed with zero text chunks")
-                } else if let last = messages.last, !last.text.isEmpty {
-                    store.appendMessage(last)
                 }
+                // streaming = false is set when the display queue drains
             } catch {
                 Log.chat.error("Stream error: \(error, privacy: .public)")
+                flushDisplay()
                 streaming = false
                 if !Task.isCancelled {
-                    // Remove empty assistant message on error
-                    if let last = messages.indices.last, messages[last].text.isEmpty {
+                    if messages.last?.text.isEmpty == true {
                         messages.removeLast()
                     }
                     self.error = error.localizedDescription
@@ -115,4 +116,67 @@ final class ChatViewModel {
             }
         }
     }
+
+    // MARK: - Throttled display (matches JS client's MessageStreamController)
+
+    private func enqueueChunk(_ chunk: String) {
+        chunkQueue.append(chunk)
+        if displayTask == nil {
+            startDisplayLoop()
+        }
+    }
+
+    private func startDisplayLoop() {
+        displayTask = Task {
+            while !Task.isCancelled {
+                guard !chunkQueue.isEmpty else {
+                    // Queue empty — check if stream is done
+                    if currentTask == nil || fullText == displayedText {
+                        finishDisplay()
+                        return
+                    }
+                    // Wait a tick for more chunks
+                    try? await Task.sleep(for: .milliseconds(50))
+                    continue
+                }
+
+                let chunk = chunkQueue.removeFirst()
+                displayedText += chunk
+
+                // Update or create the assistant message
+                if let last = messages.indices.last, messages[last].role == .assistant {
+                    messages[last].text = displayedText
+                } else {
+                    messages.append(ChatMessage(role: .assistant, text: displayedText))
+                }
+
+                // Random delay between 30-80ms per chunk for natural reading pace
+                // (shorter than the JS client's 200-400ms because iOS chunks are smaller)
+                let delay = Int.random(in: 30...80)
+                try? await Task.sleep(for: .milliseconds(delay))
+            }
+        }
+    }
+
+    private func flushDisplay() {
+        displayTask?.cancel()
+        displayTask = nil
+        displayedText = fullText
+        if !displayedText.isEmpty {
+            if let last = messages.indices.last, messages[last].role == .assistant {
+                messages[last].text = displayedText
+            } else {
+                messages.append(ChatMessage(role: .assistant, text: displayedText))
+            }
+        }
+    }
+
+    private func finishDisplay() {
+        displayTask = nil
+        streaming = false
+        // Save completed message
+        if let last = messages.last, !last.text.isEmpty {
+            store.appendMessage(last)
+        }
+    }
 }