Cache TUI flex heights across frame passes (#34348)

## Why

Sizing, rendering, and cursor placement can query the same chat widget layout
multiple times in one frame, repeatedly measuring active transcript cells.

## What changed

- Build one chat widget renderable tree per frame and reuse it for sizing,
  rendering, and cursor placement.
- Cache each flex child's desired height by width for the lifetime of that tree.
- Reuse the bottom pane's renderable directly instead of forwarding each
  renderable operation through a wrapper.

## Testing

- Verify flex layouts measure a child once across frame passes and remeasure it
  when the width changes.
- Verify a chat widget frame measures its active transcript cell once.

GitOrigin-RevId: 5ad1a6711f4011c699b5d002b13dc3319cb4db8e

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

@@ -3,7 +3,7 @@
 use super::*;
 
 impl ChatWidget {
-    pub(super) fn as_renderable(&self) -> RenderableItem<'_> {
+    pub(crate) fn as_renderable(&self) -> RenderableItem<'_> {
         let active_cell_right_reserve = self.ambient_pet_wrap_reserved_cols();
         let active_cell_renderable = match &self.transcript.active_cell {
             Some(cell) => RenderableItem::Owned(Box::new(TranscriptAreaRenderable {
@@ -48,42 +48,17 @@ impl ChatWidget {
         }
         flex.push(
             /*flex*/ 0,
-            RenderableItem::Owned(Box::new(BottomPaneComposerReserveRenderable {
-                bottom_pane: &self.bottom_pane,
-                right_reserve: active_cell_right_reserve,
-            }))
-            .inset(Insets::tlbr(
-                /*top*/ 1, /*left*/ 0, /*bottom*/ 0, /*right*/ 0,
-            )),
+            self.bottom_pane
+                .as_renderable_with_composer_right_reserve(active_cell_right_reserve)
+                .inset(Insets::tlbr(
+                    /*top*/ 1, /*left*/ 0, /*bottom*/ 0, /*right*/ 0,
+                )),
         );
         RenderableItem::Owned(Box::new(flex))
     }
-}
 
-struct BottomPaneComposerReserveRenderable<'a> {
-    bottom_pane: &'a BottomPane,
-    right_reserve: u16,
-}
-
-impl Renderable for BottomPaneComposerReserveRenderable<'_> {
-    fn render(&self, area: Rect, buf: &mut Buffer) {
-        self.bottom_pane
-            .render_with_composer_right_reserve(area, buf, self.right_reserve);
-    }
-
-    fn desired_height(&self, width: u16) -> u16 {
-        self.bottom_pane
-            .desired_height_with_composer_right_reserve(width, self.right_reserve)
-    }
-
-    fn cursor_pos(&self, area: Rect) -> Option<(u16, u16)> {
-        self.bottom_pane
-            .cursor_pos_with_composer_right_reserve(area, self.right_reserve)
-    }
-
-    fn cursor_style(&self, area: Rect) -> crossterm::cursor::SetCursorStyle {
-        self.bottom_pane
-            .cursor_style_with_composer_right_reserve(area, self.right_reserve)
+    pub(crate) fn note_rendered_width(&self, width: u16) {
+        self.last_rendered_width.set(Some(width as usize));
     }
 }
 
@@ -132,7 +107,7 @@ impl TranscriptAreaRenderable<'_> {
 impl Renderable for ChatWidget {
     fn render(&self, area: Rect, buf: &mut Buffer) {
         self.as_renderable().render(area, buf);
-        self.last_rendered_width.set(Some(area.width as usize));
+        self.note_rendered_width(area.width);
     }
 
     fn desired_height(&self, width: u16) -> u16 {

codex-rs/tui/src/chatwidget/tests/helpers.rs

@@ -215,6 +215,10 @@ pub(super) async fn make_chatwidget_manual_with_auth(
     (widget, rx, op_rx)
 }
 
+pub(crate) fn set_active_cell(chat: &mut ChatWidget, cell: Box<dyn HistoryCell>) {
+    chat.transcript.active_cell = Some(cell);
+}
+
 // ChatWidget may emit other `Op`s (e.g. history/logging updates) on the same channel; this helper
 // filters until we see a submission op.
 pub(super) fn next_submit_op(op_rx: &mut tokio::sync::mpsc::UnboundedReceiver<Op>) -> Op {