perf(tui2): reduce unnecessary redraws (#8681)

This reduces unnecessary frame scheduling in codex-tui2.

Changes:
- Gate redraw scheduling for streaming deltas when nothing visible
changes.
- Avoid a redraw feedback loop from footer transcript UI state updates.

Why:
- Streaming deltas can arrive at very high frequency; redrawing on every
delta can drive a near-constant render loop.
- BottomPane was requesting another frame after every Draw even when the
derived transcript UI state was unchanged.

Testing:
- cargo test -p codex-tui2

Manual sampling:
- sample "$(pgrep -n codex-tui2)" 3 -file
/tmp/tui2.idle.after.sample.txt
- sample "$(pgrep -n codex-tui2)" 3 -file
/tmp/tui2.streaming.after.sample.txt

Author: joshka-oai

codex-rs/tui2/src/bottom_pane/chat_composer.rs

@@ -1577,11 +1577,20 @@ impl ChatComposer {
         selection_active: bool,
         scroll_position: Option<(usize, usize)>,
         copy_selection_key: KeyBinding,
-    ) {
+    ) -> bool {
+        if self.transcript_scrolled == scrolled
+            && self.transcript_selection_active == selection_active
+            && self.transcript_scroll_position == scroll_position
+            && self.transcript_copy_selection_key == copy_selection_key
+        {
+            return false;
+        }
+
         self.transcript_scrolled = scrolled;
         self.transcript_selection_active = selection_active;
         self.transcript_scroll_position = scroll_position;
         self.transcript_copy_selection_key = copy_selection_key;
+        true
     }
 
     fn sync_popups(&mut self) {

codex-rs/tui2/src/bottom_pane/mod.rs

@@ -389,13 +389,15 @@ impl BottomPane {
         scroll_position: Option<(usize, usize)>,
         copy_selection_key: crate::key_hint::KeyBinding,
     ) {
-        self.composer.set_transcript_ui_state(
+        let updated = self.composer.set_transcript_ui_state(
             scrolled,
             selection_active,
             scroll_position,
             copy_selection_key,
         );
-        self.request_redraw();
+        if updated {
+            self.request_redraw();
+        }
     }
 
     /// Show a generic list selection view with the provided items.

codex-rs/tui2/src/chatwidget.rs

@@ -968,6 +968,7 @@ impl ChatWidget {
             if let Some(cell) = cell {
                 self.bottom_pane.hide_status_indicator();
                 self.add_boxed_history(cell);
+                self.request_redraw();
             }
             if is_idle {
                 self.app_event_tx.send(AppEvent::StopCommitAnimation);
@@ -1009,6 +1010,7 @@ impl ChatWidget {
     #[inline]
     fn handle_streaming_delta(&mut self, delta: String) {
         // Before streaming agent content, flush any active exec cell group.
+        let mut needs_redraw = self.active_cell.is_some();
         self.flush_active_cell();
 
         if self.stream_controller.is_none() {
@@ -1019,6 +1021,7 @@ impl ChatWidget {
                     .map(super::status_indicator_widget::StatusIndicatorWidget::elapsed_seconds);
                 self.add_to_history(history_cell::FinalMessageSeparator::new(elapsed_seconds));
                 self.needs_final_message_separator = false;
+                needs_redraw = true;
             }
             self.stream_controller = Some(StreamController::new(
                 self.last_rendered_width.get().map(|w| w.saturating_sub(2)),
@@ -1029,7 +1032,9 @@ impl ChatWidget {
         {
             self.app_event_tx.send(AppEvent::StartCommitAnimation);
         }
-        self.request_redraw();
+        if needs_redraw {
+            self.request_redraw();
+        }
     }
 
     pub(crate) fn handle_exec_end_now(&mut self, ev: ExecCommandEndEvent) {