refactor(tui/history): add history_*_and_maybe_merge API and adopt in exec flow

- Add  and
- Use in exec completion paths to encapsulate merge heuristics
- Continue to centralize invalidation/redraw in helpers

Validated with ./build-fast.sh (no warnings).

codex-rs/tui/src/chatwidget.rs

@@ -1592,6 +1592,19 @@ impl ChatWidget<'_> {
         }
     }
 
+    // History merge API: consolidates exec/tool merge logic
+    fn history_push_and_maybe_merge(&mut self, cell: impl HistoryCell + 'static) {
+        // For push, add_to_history already contains merge heuristics for Exec/Tool cells.
+        self.add_to_history(cell);
+    }
+
+    fn history_replace_and_maybe_merge(&mut self, idx: usize, cell: Box<dyn HistoryCell>) {
+        // Replace at index, then attempt standard exec merge with previous cell.
+        self.history_replace_at(idx, cell);
+        // Merge only if the new cell is an Exec with output (completed) or a MergedExec.
+        crate::chatwidget::exec_tools::try_merge_completed_exec_at(self, idx);
+    }
+
     /// Clean up faded-out animation cells
     fn process_animation_cleanup(&mut self) {
         // With trait-based cells, we can't easily detect and clean up specific cell types

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

@@ -428,13 +428,8 @@ pub(super) fn handle_exec_end_now(chat: &mut ChatWidget<'_>, ev: ExecCommandEndE
                 })
                 .unwrap_or(false);
             if is_match {
-                if let Some(c) = completed_opt.take() { chat.history_replace_at(idx, Box::new(c)); }
-                let pre_len = chat.history_cells.len();
+                if let Some(c) = completed_opt.take() { chat.history_replace_and_maybe_merge(idx, Box::new(c)); }
                 replaced = true;
-                try_merge_completed_exec_at(chat, idx);
-                if chat.history_cells.len() == pre_len {
-                    chat.request_redraw();
-                }
             }
         }
         if !replaced {
@@ -446,17 +441,14 @@ pub(super) fn handle_exec_end_now(chat: &mut ChatWidget<'_>, ev: ExecCommandEndE
                 }
             }
             if let Some(i) = found {
-                if let Some(c) = completed_opt.take() { chat.history_replace_at(i, Box::new(c)); }
-                let pre_len = chat.history_cells.len();
+                if let Some(c) = completed_opt.take() { chat.history_replace_and_maybe_merge(i, Box::new(c)); }
                 replaced = true;
-                try_merge_completed_exec_at(chat, i);
-                if chat.history_cells.len() == pre_len { chat.request_redraw(); }
             }
         }
     }
 
     if !replaced {
-                if let Some(c) = completed_opt.take() { chat.history_push(c); }
+                if let Some(c) = completed_opt.take() { chat.history_push_and_maybe_merge(c); }
     }
 
     if exit_code == 0 {