codex: address PR review feedback (#16803)

Backfill a missing reasoning-summary prefix from the completed item snapshot when late suffix deltas streamed normally after earlier orphan deltas were dropped.

codex-rs/tui/src/chatwidget.rs

@@ -6016,10 +6016,13 @@ impl ChatWidget {
             ThreadItem::Reasoning {
                 summary, content, ..
             } => {
-                let should_backfill_reasoning = from_replay
-                    || (self.reasoning_buffer.trim().is_empty()
-                        && self.full_reasoning_buffer.trim().is_empty());
-                if should_backfill_reasoning {
+                let mut completed_reasoning = summary.join("");
+                if self.config.show_raw_agent_reasoning {
+                    completed_reasoning.push_str(&content.join(""));
+                }
+                let streamed_reasoning =
+                    format!("{}{}", self.full_reasoning_buffer, self.reasoning_buffer);
+                if from_replay || streamed_reasoning.trim().is_empty() {
                     for delta in summary {
                         self.on_agent_reasoning_delta(delta);
                     }
@@ -6028,6 +6031,17 @@ impl ChatWidget {
                             self.on_agent_reasoning_delta(delta);
                         }
                     }
+                } else if !completed_reasoning.is_empty()
+                    && completed_reasoning != streamed_reasoning
+                    && let Some(missing_prefix) = completed_reasoning
+                        .strip_suffix(&streamed_reasoning)
+                        .filter(|missing_prefix| !missing_prefix.is_empty())
+                {
+                    // If early summary deltas were dropped before the item became active, prepend
+                    // the missing prefix from the completed item snapshot and keep the streamed
+                    // suffix so we still avoid double-rendering.
+                    self.full_reasoning_buffer =
+                        format!("{missing_prefix}{}", self.full_reasoning_buffer);
                 }
                 self.on_agent_reasoning_final();
             }

codex-rs/tui/src/chatwidget/snapshots/codex_tui__chatwidget__tests__live_reasoning_item_completed_backfills_missing_prefix_before_streamed_suffix.snap

@@ -0,0 +1,5 @@
+---
+source: tui/src/chatwidget/tests/history_replay.rs
+expression: rendered
+---
+• prefix suffix

codex-rs/tui/src/chatwidget/tests/history_replay.rs

@@ -770,6 +770,62 @@ async fn live_reasoning_item_completed_renders_summary_without_prior_delta() {
     );
 }
 
+#[tokio::test]
+async fn live_reasoning_item_completed_backfills_missing_prefix_before_streamed_suffix() {
+    let (mut chat, mut rx, _op_rx) = make_chatwidget_manual(/*model_override*/ None).await;
+    chat.show_welcome_banner = false;
+
+    chat.handle_server_notification(
+        ServerNotification::TurnStarted(TurnStartedNotification {
+            thread_id: "thread-1".to_string(),
+            turn: AppServerTurn {
+                id: "turn-1".to_string(),
+                items: Vec::new(),
+                status: AppServerTurnStatus::InProgress,
+                error: None,
+            },
+        }),
+        /*replay_kind*/ None,
+    );
+    let _ = drain_insert_history(&mut rx);
+
+    chat.handle_server_notification(
+        ServerNotification::ReasoningSummaryTextDelta(ReasoningSummaryTextDeltaNotification {
+            thread_id: "thread-1".to_string(),
+            turn_id: "turn-1".to_string(),
+            item_id: "reasoning-1".to_string(),
+            delta: "suffix".to_string(),
+            summary_index: 0,
+        }),
+        /*replay_kind*/ None,
+    );
+
+    chat.handle_server_notification(
+        ServerNotification::ItemCompleted(ItemCompletedNotification {
+            thread_id: "thread-1".to_string(),
+            turn_id: "turn-1".to_string(),
+            item: AppServerThreadItem::Reasoning {
+                id: "reasoning-1".to_string(),
+                summary: vec!["prefix suffix".to_string()],
+                content: Vec::new(),
+            },
+        }),
+        /*replay_kind*/ None,
+    );
+
+    let rendered = match rx.try_recv() {
+        Ok(AppEvent::InsertHistoryCell(cell)) => {
+            lines_to_single_string(&cell.transcript_lines(/*width*/ 80))
+        }
+        other => panic!("expected InsertHistoryCell, got {other:?}"),
+    };
+    assert_eq!(rendered.matches("prefix suffix").count(), 1);
+    assert_chatwidget_snapshot!(
+        "live_reasoning_item_completed_backfills_missing_prefix_before_streamed_suffix",
+        rendered
+    );
+}
+
 #[tokio::test]
 async fn replayed_turn_started_does_not_mark_task_running() {
     let (mut chat, _rx, _op_rx) = make_chatwidget_manual(/*model_override*/ None).await;