Reland "refactor transcript view to handle HistoryCells" (#3753)

Reland of #3538

codex-rs/core/tests/suite/compact_resume_fork.rs

@@ -74,7 +74,7 @@ async fn compact_resume_and_fork_preserve_model_history_view() {
         "compact+resume test expects resumed path {resumed_path:?} to exist",
     );
 
-    let forked = fork_conversation(&manager, &config, resumed_path, 1).await;
+    let forked = fork_conversation(&manager, &config, resumed_path, 2).await;
     user_turn(&forked, "AFTER_FORK").await;
 
     // 3. Capture the requests to the model and validate the history slices.
@@ -100,17 +100,15 @@ async fn compact_resume_and_fork_preserve_model_history_view() {
         "after-resume input should have at least as many items as after-compact",
     );
     assert_eq!(compact_arr.as_slice(), &resume_arr[..compact_arr.len()]);
-    eprint!(
-        "len of compact: {}, len of fork: {}",
-        compact_arr.len(),
-        fork_arr.len()
-    );
-    eprintln!("input_after_fork:{}", json!(input_after_fork));
+
     assert!(
         compact_arr.len() <= fork_arr.len(),
         "after-fork input should have at least as many items as after-compact",
     );
-    assert_eq!(compact_arr.as_slice(), &fork_arr[..compact_arr.len()]);
+    assert_eq!(
+        &compact_arr.as_slice()[..compact_arr.len()],
+        &fork_arr[..compact_arr.len()]
+    );
 
     let prompt = requests[0]["instructions"]
         .as_str()
@@ -824,14 +822,15 @@ async fn resume_conversation(
     conversation
 }
 
+#[cfg(test)]
 async fn fork_conversation(
     manager: &ConversationManager,
     config: &Config,
     path: std::path::PathBuf,
-    back_steps: usize,
+    nth_user_message: usize,
 ) -> Arc<CodexConversation> {
     let NewConversation { conversation, .. } = manager
-        .fork_conversation(back_steps, config.clone(), path)
+        .fork_conversation(nth_user_message, config.clone(), path)
         .await
         .expect("fork conversation");
     conversation

codex-rs/core/tests/suite/fork_conversation.rs

@@ -5,6 +5,8 @@ use codex_core::ModelProviderInfo;
 use codex_core::NewConversation;
 use codex_core::ResponseItem;
 use codex_core::built_in_model_providers;
+use codex_core::content_items_to_text;
+use codex_core::is_session_prefix_message;
 use codex_core::protocol::ConversationPathResponseEvent;
 use codex_core::protocol::EventMsg;
 use codex_core::protocol::InputItem;
@@ -104,13 +106,16 @@ async fn fork_conversation_twice_drops_to_first_message() {
         items
     };
 
-    // Compute expected prefixes after each fork by truncating base rollout at nth-from-last user input.
+    // Compute expected prefixes after each fork by truncating base rollout
+    // strictly before the nth user input (0-based).
     let base_items = read_items(&base_path);
     let find_user_input_positions = |items: &[RolloutItem]| -> Vec<usize> {
         let mut pos = Vec::new();
         for (i, it) in items.iter().enumerate() {
             if let RolloutItem::ResponseItem(ResponseItem::Message { role, content, .. }) = it
                 && role == "user"
+                && content_items_to_text(content)
+                    .is_some_and(|text| !is_session_prefix_message(&text))
             {
                 // Consider any user message as an input boundary; recorder stores both EventMsg and ResponseItem.
                 // We specifically look for input items, which are represented as ContentItem::InputText.
@@ -126,11 +131,8 @@ async fn fork_conversation_twice_drops_to_first_message() {
     };
     let user_inputs = find_user_input_positions(&base_items);
 
-    // After dropping last user input (n=1), cut strictly before that input if present, else empty.
-    let cut1 = user_inputs
-        .get(user_inputs.len().saturating_sub(1))
-        .copied()
-        .unwrap_or(0);
+    // After cutting at nth user input (n=1 → second user message), cut strictly before that input.
+    let cut1 = user_inputs.get(1).copied().unwrap_or(0);
     let expected_after_first: Vec<RolloutItem> = base_items[..cut1].to_vec();
 
     // After dropping again (n=1 on fork1), compute expected relative to fork1's rollout.
@@ -161,12 +163,12 @@ async fn fork_conversation_twice_drops_to_first_message() {
         serde_json::to_value(&expected_after_first).unwrap()
     );
 
-    // Fork again with n=1 → drops the (new) last user message, leaving only the first.
+    // Fork again with n=0 → drops the (new) last user message, leaving only the first.
     let NewConversation {
         conversation: codex_fork2,
         ..
     } = conversation_manager
-        .fork_conversation(1, config_for_fork.clone(), fork1_path.clone())
+        .fork_conversation(0, config_for_fork.clone(), fork1_path.clone())
         .await
         .expect("fork 2");