fix(ui): prevent content vanishing on streaming completion

Three changes to fix scroll position loss when streaming ends:

1. Eliminate double completeStreamingMessage - ChatWidget now checks
   if message is still streaming before calling completeStreamingMessage
   again; if already completed by orchestrator, uses updateMessage to
   add performance metrics without triggering full finalization

2. Streaming completion scroll uses scroll-bottom-anchor instead of
   message ID with .bottom anchor, and delay increased from 50ms to
   150ms to allow layout to settle after ProgressView removal and
   metrics addition

3. Remove content trimming in completeStreamingMessage to prevent
   cache invalidation cascade (content hash change -> getCachedClean
   miss -> displayedContent update -> MarkdownContentView re-parse)

Author: fewtarius

Sources/ConversationEngine/ConversationMessageBus.swift

@@ -260,7 +260,7 @@ public class ConversationMessageBus: ObservableObject {
         let updated = EnhancedMessage(
             id: current.id,
             type: current.type,
-            content: current.content.trimmingCharacters(in: .whitespacesAndNewlines), // Trim on completion
+            content: current.content, // Preserve streamed content as-is to avoid cache invalidation
             contentParts: current.contentParts,
             isFromUser: current.isFromUser,
             timestamp: current.timestamp,
@@ -305,7 +305,9 @@ public class ConversationMessageBus: ObservableObject {
         contentParts: [MessageContentPart]? = nil,
         toolCalls: [SimpleToolCall]? = nil,
         status: ToolStatus? = nil,
-        duration: TimeInterval? = nil
+        duration: TimeInterval? = nil,
+        performanceMetrics: MessagePerformanceMetrics? = nil,
+        processingTime: TimeInterval? = nil
     ) {
         guard let index = messageCache[id] else {
             logger.error("UPDATE_MESSAGE: Message not found id=\(id.uuidString.prefix(8))")
@@ -331,10 +333,10 @@ public class ConversationMessageBus: ObservableObject {
             toolMetadata: current.toolMetadata,
             toolCalls: toolCalls ?? current.toolCalls,
             toolCallId: current.toolCallId,
-            processingTime: current.processingTime,
+            processingTime: processingTime ?? current.processingTime,
             reasoningContent: current.reasoningContent,
             showReasoning: current.showReasoning,
-            performanceMetrics: current.performanceMetrics,
+            performanceMetrics: performanceMetrics ?? current.performanceMetrics,
             isStreaming: current.isStreaming,
             isToolMessage: current.isToolMessage,
             githubCopilotResponseId: current.githubCopilotResponseId,

Sources/UserInterface/Chat/ChatWidget.swift

@@ -1279,14 +1279,15 @@ public struct ChatWidget: View {
                 performThrottledScroll(proxy: proxy, to: lastMessage)
             }
             .onChange(of: messages.last?.isStreaming) { oldValue, newValue in
-                /// Streaming completion correction: when streaming ends, the layout
-                /// recomputes (fixedSize kicks in for the full message height).
-                /// Fire a delayed scroll to keep content visible after relayout.
+                /// Streaming completion scroll: when streaming ends, layout recomputes
+                /// (ProgressView removed, metrics added, content re-parsed).
+                /// Scroll to bottom anchor after layout settles to keep content visible.
                 if oldValue == true && newValue == false {
-                    guard scrollLockEnabled, let lastMessage = messages.last else { return }
+                    guard scrollLockEnabled else { return }
                     Task { @MainActor in
-                        try? await Task.sleep(for: .milliseconds(50))
-                        proxy.scrollTo(lastMessage.id.uuidString, anchor: .bottom)
+                        /// 150ms allows layout to settle after streaming->complete transition
+                        try? await Task.sleep(for: .milliseconds(150))
+                        proxy.scrollTo("scroll-bottom-anchor", anchor: .bottom)
                     }
                 }
             }
@@ -4037,13 +4038,23 @@ public struct ChatWidget: View {
                         messageMetrics = nil
                     }
 
-                    /// FIXED: Use MessageBus to finalize streaming message
-                    /// completeStreamingMessage marks isStreaming=false, trims content, adds metrics
-                    activeConversation?.messageBus?.completeStreamingMessage(
-                        id: msgId,
-                        performanceMetrics: messageMetrics,
-                        processingTime: processingTime
-                    )
+                    /// Only finalize if still streaming (orchestrator may have already completed it)
+                    /// Calling completeStreamingMessage twice causes double layout disruption
+                    let currentMessage = messages[index]
+                    if currentMessage.isStreaming {
+                        activeConversation?.messageBus?.completeStreamingMessage(
+                            id: msgId,
+                            performanceMetrics: messageMetrics,
+                            processingTime: processingTime
+                        )
+                    } else if messageMetrics != nil {
+                        /// Message already completed by orchestrator - just add metrics
+                        activeConversation?.messageBus?.updateMessage(
+                            id: msgId,
+                            performanceMetrics: messageMetrics,
+                            processingTime: processingTime
+                        )
+                    }
 
                     let existing = messages[index]  // Read for voice/workflow checks
                     logger.debug("MESSAGE_LIFECYCLE: Finalized message \(assistantMessageId?.uuidString.prefix(8) ?? "nil") with \(existing.content.count) chars (count still: \(messages.count))")