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

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.