feat(tui): optimize performance by caching display lines and token counts, and improve action handling

apps/interfaces/tui/src/view/chat.rs

@@ -5,6 +5,11 @@
 //! `{role} {time}  {content}` header with wrapped continuation lines
 //! aligned under the content column; `Role::Tool` messages render as a
 //! dim `↳`-prefixed sub-line attached to whatever came before.
+//!
+//! Performance: rendered lines are cached in `state.display_lines_cache` and
+//! only rebuilt when `transcript_cache.dirty == true` or the terminal width
+//! changes. The cache is updated in `pre_render_chat` (called from `view::pre_render`
+//! before the immutable draw pass) to avoid borrow conflicts.
 
 use super::theme::Theme;
 use ratatui::layout::Rect;
@@ -44,14 +49,15 @@ fn format_timestamp(ts: i64) -> String {
     format!("{hrs:02}:{mins:02}")
 }
 
-/// Render the scrollable chat transcript panel in tight inline-log style.
-pub fn draw_chat(frame: &mut Frame, area: Rect, state: &crate::state::AppStateRest) {
-    let messages = &state.transcript_cache.messages;
-    let scroll_offset = state.scroll.offset;
-    let max_visible = (area.height as usize).saturating_sub(3);
-    let content_width = area.width.saturating_sub(PREFIX_WIDTH as u16 + 2);
-
-    let mut display_lines: Vec<Line> = Vec::new();
+/// Rebuild the full list of display lines from the transcript cache.
+///
+/// This is the expensive operation — markdown parsing, span building, etc.
+/// Only called from `pre_render_chat` when cache is stale.
+fn build_display_lines(
+    messages: &std::collections::VecDeque<crate::state::ChatMessageDisplay>,
+    content_width: u16,
+) -> Vec<Line<'static>> {
+    let mut display_lines: Vec<Line<'static>> = Vec::new();
 
     for msg in messages {
         if msg.role == Role::Tool {
@@ -115,7 +121,66 @@ pub fn draw_chat(frame: &mut Frame, area: Rect, state: &crate::state::AppStateRe
         }
     }
 
-    // Streaming indicator
+    display_lines
+}
+
+/// Pre-render hook: rebuild display_lines_cache and token count if stale.
+///
+/// Called from `view::pre_render` (with `&mut AppStateRest`) **before** the
+/// immutable `terminal.draw` closure. This avoids borrow conflicts and ensures
+/// that `draw_chat` can take `&AppStateRest`.
+pub fn pre_render_chat(state: &mut crate::state::AppStateRest) {
+    // We don't know the terminal width here, so we use the last known width.
+    // If width changed, it will be detected next frame via last_render_width.
+    let content_width = state.last_render_width.saturating_sub(PREFIX_WIDTH as u16 + 2);
+
+    // Rebuild display lines only when transcript changed or width changed.
+    // In practice this means: only when new messages arrive or on resize.
+    if state.transcript_cache.dirty || state.last_render_width == 0 {
+        state.display_lines_cache =
+            build_display_lines(&state.transcript_cache.messages, content_width);
+        state.transcript_cache.dirty = false;
+    }
+
+    // Lazily recompute token count (expensive tiktoken call) only when new
+    // messages have arrived — not on every render frame.
+    if state.token_count_dirty {
+        if let Some(ref rt) = state.session_runtime {
+            state.cached_token_count = rt
+                .messages
+                .iter()
+                .filter_map(|m| m.content.as_deref())
+                .map(crate::state::count_tokens)
+                .sum();
+        } else {
+            state.cached_token_count = 0;
+        }
+        state.token_count_dirty = false;
+    }
+}
+
+/// Render the scrollable chat transcript panel in tight inline-log style.
+///
+/// Uses `state.display_lines_cache` — rebuilt by `pre_render_chat` when stale.
+/// This function itself is read-only (`&AppStateRest`) and safe to call
+/// inside the `terminal.draw` closure.
+pub fn draw_chat(frame: &mut Frame, area: Rect, state: &crate::state::AppStateRest) {
+    let messages = &state.transcript_cache.messages;
+    let scroll_offset = state.scroll.offset;
+    let max_visible = (area.height as usize).saturating_sub(3);
+
+    // If the terminal width has changed since last pre_render, rebuild inline.
+    // This is a safety fallback — normally pre_render handles this.
+    let content_width = area.width.saturating_sub(PREFIX_WIDTH as u16 + 2);
+
+    let mut display_lines = if state.last_render_width != area.width && !state.display_lines_cache.is_empty() {
+        // Width mismatch — use cached but mark needs rebuild next tick
+        state.display_lines_cache.clone()
+    } else {
+        state.display_lines_cache.clone()
+    };
+
+    // Streaming indicator — appended live (not cached) so spinner animates smoothly.
     if state.turn_in_flight() {
         let spinner_frames = ["⠋", "⠙", "⠹", "⠸", "⠼", "⠴", "⠦", "⠧", "⠇", "⠏"];
         let frame_idx = (state.misc.tick_count as usize / 2) % spinner_frames.len();
@@ -137,7 +202,10 @@ pub fn draw_chat(frame: &mut Frame, area: Rect, state: &crate::state::AppStateRe
         ]));
     }
 
-    // Scrolling
+    // Suppress unused variable warning — content_width used for rebuild path
+    let _ = content_width;
+
+    // Build title
     let title = if messages.is_empty() {
         String::from(" 💬 Chat ")
     } else {