Only show the assistant activity heading when there's activity to reveal (#4424)

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

.changeset/collapsible-reasoning-heading.md

@@ -0,0 +1,5 @@
+---
+"gitbook": patch
+---
+
+Assistant: the "Explored briefly" activity heading no longer appears when there's nothing to show. It now renders only when the answer is preceded by a real preamble or one or more tool calls, so a simple answer with an empty reasoning step no longer surfaces an empty collapsible.

packages/gitbook/src/components/AI/useAIChat.tsx

@@ -863,12 +863,22 @@ function updateAIChatMessageActivity(
             return {
                 ...activity,
                 currentPhase: event.phase,
-                hasCommentary:
-                    activity.hasCommentary || event.phase === AIMessageStepPhase.Commentary,
                 hasFinalAnswer:
                     activity.hasFinalAnswer || event.phase === AIMessageStepPhase.FinalAnswer,
             };
         }
+        case 'response_document': {
+            // A commentary phase can start without ever producing anything visible. Only a
+            // commentary step that emits document content is a real preamble worth collapsing
+            // behind the activity heading, so flag it here rather than on phase start.
+            return {
+                ...activity,
+                hasCommentary:
+                    activity.hasCommentary ||
+                    (activity.currentPhase === AIMessageStepPhase.Commentary &&
+                        event.blocks.length > 0),
+            };
+        }
         case 'response_tool_call': {
             return {
                 ...activity,