Keep fork lineage ahead of paginated transcript history (#54044)

## Why

Loading older history could place transcript entries before the `Thread forked from` notice, separating fork lineage from the session header.

## What changed

Treat fork lineage as a session prefix in both the TUI transcript and transcript overlay so older pages are inserted after it.

## Testing

Add regression tests for repeated history prepends in both paths, verifying that fork lineage stays first.

GitOrigin-RevId: 71497fb7ffa992c72ee08617d318aa3119e71275

codex-rs/tui/src/app/history_pagination.rs

@@ -6,9 +6,8 @@ use std::ops::Range;
 use super::*;
 use crate::app_server_session::HISTORY_ITEM_PAGE_LIMIT;
 use crate::app_server_session::thread_items_page_params;
-use crate::history_cell::SessionHeaderHistoryCell;
-use crate::history_cell::SessionInfoCell;
 use crate::history_cell::UserHistoryCell;
+use crate::history_cell::is_session_history_prefix_cell;
 use crate::pager_overlay::TranscriptHistoryState;
 use crate::thread_transcript::RawReasoningVisibility;
 use crate::thread_transcript::thread_items_to_transcript_cells;
@@ -265,10 +264,7 @@ impl App {
         } else {
             self.transcript_cells
                 .iter()
-                .rposition(|cell| {
-                    cell.as_any().is::<SessionInfoCell>()
-                        || cell.as_any().is::<SessionHeaderHistoryCell>()
-                })
+                .rposition(|cell| is_session_history_prefix_cell(cell.as_ref()))
                 .map_or(/*default*/ 0, |index| index.saturating_add(/*rhs*/ 1))
         };
         let inserted = index..index + cells.len();

codex-rs/tui/src/app/history_pagination_tests.rs

@@ -4,6 +4,7 @@ use super::*;
 use crate::app::test_support::make_test_app;
 use crate::history_cell::ComputerActivityCell;
 use crate::history_cell::FinalMessageSeparator;
+use crate::history_cell::ForkedThreadHistoryCell;
 use codex_app_server_protocol::TurnItemsView;
 use codex_app_server_protocol::UserInput;
 use pretty_assertions::assert_eq;
@@ -60,6 +61,45 @@ fn user_cell(message: &str) -> Arc<dyn HistoryCell> {
     })
 }
 
+#[tokio::test]
+async fn fork_lineage_stays_before_repeated_older_history_pages() {
+    let mut app = make_test_app().await;
+    app.transcript_cells = vec![
+        Arc::new(ForkedThreadHistoryCell(
+            crate::history_cell::PlainHistoryCell::new(vec!["Thread forked from parent".into()]),
+        )),
+        Arc::new(crate::history_cell::PlainHistoryCell::new(vec![
+            "recent history".into(),
+        ])),
+    ];
+
+    app.prepend_older_transcript_cells(vec![Arc::new(crate::history_cell::PlainHistoryCell::new(
+        vec!["older history".into()],
+    ))]);
+    app.prepend_older_transcript_cells(vec![Arc::new(crate::history_cell::PlainHistoryCell::new(
+        vec!["oldest history".into()],
+    ))]);
+
+    let rendered = app
+        .transcript_cells
+        .iter()
+        .flat_map(|cell| cell.display_lines(/*width*/ 80))
+        .map(|line| {
+            line.spans
+                .into_iter()
+                .map(|span| span.content.into_owned())
+                .collect::<String>()
+        })
+        .collect::<Vec<_>>()
+        .join("\n");
+    insta::assert_snapshot!(rendered, @r###"
+    Thread forked from parent
+    oldest history
+    older history
+    recent history
+    "###);
+}
+
 #[test]
 fn repeated_prompt_matching_removes_only_the_hidden_occurrence() {
     let cells = vec![user_cell("repeat"), user_cell("other"), user_cell("repeat")];

codex-rs/tui/src/chatwidget/session_flow.rs

@@ -3,6 +3,7 @@
 //! checks the previous model when it applies the final selection.
 
 use super::*;
+use crate::history_cell::ForkedThreadHistoryCell;
 use crate::style::accent_color;
 
 impl ChatWidget {
@@ -310,7 +311,7 @@ impl ChatWidget {
             .into()
         };
         self.app_event_tx.send(AppEvent::InsertHistoryCell(Box::new(
-            PlainHistoryCell::new(vec![line]),
+            ForkedThreadHistoryCell(PlainHistoryCell::new(vec![line])),
         )));
     }
 

codex-rs/tui/src/history_cell/session.rs

@@ -103,6 +103,20 @@ impl HistoryCell for SessionNoticeCell {
     }
 }
 
+/// Fork lineage that stays with the session header when older history is prepended.
+#[derive(Debug)]
+pub(crate) struct ForkedThreadHistoryCell(pub(crate) PlainHistoryCell);
+
+impl HistoryCell for ForkedThreadHistoryCell {
+    fn display_lines(&self, width: u16) -> Vec<Line<'static>> {
+        self.0.display_lines(width)
+    }
+
+    fn raw_lines(&self) -> Vec<Line<'static>> {
+        self.0.raw_lines()
+    }
+}
+
 #[derive(Debug)]
 pub struct SessionInfoCell(CompositeHistoryCell);
 
@@ -165,6 +179,13 @@ impl HistoryCell for SessionInfoCell {
     }
 }
 
+/// Session-level cells that stay ahead of every loaded transcript page.
+pub(crate) fn is_session_history_prefix_cell(cell: &dyn HistoryCell) -> bool {
+    cell.as_any().is::<SessionInfoCell>()
+        || cell.as_any().is::<SessionHeaderHistoryCell>()
+        || cell.as_any().is::<ForkedThreadHistoryCell>()
+}
+
 #[expect(
     clippy::too_many_arguments,
     reason = "keep local preferences separate while the legacy Config parameter is still required"

codex-rs/tui/src/pager_overlay/transcript.rs

@@ -5,8 +5,7 @@
 
 use super::*;
 use crate::history_cell::HistoryRenderMode;
-use crate::history_cell::SessionHeaderHistoryCell;
-use crate::history_cell::SessionInfoCell;
+use crate::history_cell::is_session_history_prefix_cell;
 use crate::keymap::RuntimeKeymap;
 use crate::motion::MotionMode;
 use crate::transcript_view::TranscriptBookmark;
@@ -313,10 +312,7 @@ impl TranscriptOverlay {
         let index = self
             .cells
             .iter()
-            .rposition(|cell| {
-                cell.as_any().is::<SessionInfoCell>()
-                    || cell.as_any().is::<SessionHeaderHistoryCell>()
-            })
+            .rposition(|cell| is_session_history_prefix_cell(cell.as_ref()))
             .map_or(/*default*/ 0, |index| index + 1);
         let added = cells.len();
         self.cells.splice(index..index, cells);

codex-rs/tui/src/pager_overlay/transcript_tests.rs

@@ -60,6 +60,29 @@ fn scrolling_prepend_and_resize_preserve_reading_content() {
     assert!(!overlay.is_scrolled_to_bottom());
 }
 
+#[test]
+fn repeated_prepend_keeps_fork_lineage_first() {
+    let lineage: Arc<dyn HistoryCell> = Arc::new(history_cell::ForkedThreadHistoryCell(
+        history_cell::PlainHistoryCell::new(vec!["fork lineage".into()]),
+    ));
+    let mut overlay = transcript_overlay(vec![
+        Arc::clone(&lineage),
+        Arc::new(TestCell {
+            lines: vec!["recent".into()],
+        }),
+    ]);
+
+    let older = overlay.prepend(vec![Arc::new(TestCell {
+        lines: vec!["older".into()],
+    })]);
+    let oldest = overlay.prepend(vec![Arc::new(TestCell {
+        lines: vec!["oldest".into()],
+    })]);
+
+    assert_eq!((older, oldest), (1, 1));
+    assert!(Arc::ptr_eq(&overlay.cells[0], &lineage));
+}
+
 #[tokio::test]
 async fn activity_hint_uses_the_configured_pager_jump_binding() -> Result<()> {
     let mut tui = crate::tui::test_support::make_test_tui()?;