Skip syntax highlighting for lines over 4 KiB (#34796)

## Why

Very long individual lines can make syntax highlighting consume excessive CPU or memory even when the overall input remains below the existing size and line-count limits.

## What changed

- Fall back to plain, unstyled text when any input line exceeds 4 KiB.
- Keep the existing 512 KiB total-size and 10,000-line guardrails.

## Testing

- Cover highlighting fallback and text preservation for a long single-line Bash command.
- Snapshot rendering of a command that exceeds the per-line limit.

GitOrigin-RevId: ba02997331a294d8a32ca89ee06acb12dd3bbd83

codex-rs/tui/src/history_cell/snapshots/codex_tui__history_cell__tests__single_line_command_over_highlight_limit_uses_plain_text_fallback.snap

@@ -0,0 +1,9 @@
+---
+source: tui/src/history_cell/tests.rs
+expression: rendered
+---
+• Ran AAAAAAAAAAAAAAAAAA
+  │ AAAAAAAAAAAAAAAAAA
+  │ AAAAAAAAAAAAAAAAAA
+  │ … +225 lines
+  └ (no output)

codex-rs/tui/src/history_cell/tests.rs

@@ -7,6 +7,7 @@ use crate::exec_cell::ExecCell;
 use crate::legacy_core::config::Config;
 use crate::legacy_core::config::ConfigBuilder;
 use crate::line_truncation::line_width;
+use crate::render::highlight::MAX_HIGHLIGHT_LINE_BYTES;
 use crate::session_state::ThreadSessionState;
 use crate::wrapping::word_wrap_lines;
 use codex_app_server_protocol::AskForApproval;
@@ -1861,6 +1862,30 @@ fn single_line_command_wraps_with_four_space_continuation() {
     insta::assert_snapshot!(rendered);
 }
 
+#[test]
+fn single_line_command_over_highlight_limit_uses_plain_text_fallback() {
+    let call_id = "c1".to_string();
+    let base64_like = "A".repeat(MAX_HIGHLIGHT_LINE_BYTES + 1);
+    let mut cell = ExecCell::new(
+        ExecCall {
+            call_id: call_id.clone(),
+            command: vec!["bash".into(), "-lc".into(), base64_like],
+            parsed: Vec::new(),
+            output: None,
+            source: ExecCommandSource::Agent,
+            start_time: Some(Instant::now()),
+            duration: None,
+            interaction_input: None,
+        },
+        /*animations_enabled*/ true,
+    );
+    cell.complete_call(&call_id, CommandOutput::default(), Duration::from_millis(1));
+
+    let rendered = render_lines(&cell.display_lines(/*width*/ 24)).join("\n");
+
+    insta::assert_snapshot!(rendered);
+}
+
 #[test]
 fn multiline_command_without_wrap_uses_branch_then_eight_spaces() {
     let call_id = "c1".to_string();

codex-rs/tui/src/render/highlight.rs

@@ -18,9 +18,10 @@
 //! swap/snapshot the theme for live preview.  All highlighting functions read
 //! the theme via `theme_lock()`.
 //!
-//! **Guardrails:** inputs exceeding 512 KB or 10 000 lines are rejected early
-//! (returns `None`) to prevent pathological CPU/memory usage.  Callers must
-//! fall back to plain unstyled text.
+//! **Guardrails:** inputs exceeding 512 KB or 10 000 lines, or containing an
+//! individual line longer than 4 KiB, are rejected early (returns `None`) to
+//! prevent pathological CPU/memory usage.  Callers must fall back to plain
+//! unstyled text.
 
 use ratatui::style::Color as RtColor;
 use ratatui::style::Modifier;
@@ -581,6 +582,9 @@ const MAX_HIGHLIGHT_BYTES: usize = 512 * 1024;
 /// Skip highlighting for inputs with more than 10,000 lines.
 const MAX_HIGHLIGHT_LINES: usize = 10_000;
 
+/// Skip highlighting when an individual line is longer than 4 KiB.
+pub(crate) const MAX_HIGHLIGHT_LINE_BYTES: usize = 4 * 1024;
+
 /// Check whether an input exceeds the safe highlighting limits.
 ///
 /// Callers that highlight content in a loop (e.g. per diff-line) should
@@ -611,7 +615,12 @@ fn highlight_to_line_spans_with_theme(
     // Bail out early for oversized inputs to avoid excessive resource usage.
     // Count actual lines (not newline bytes) to avoid an off-by-one when
     // the input does not end with a newline.
-    if code.len() > MAX_HIGHLIGHT_BYTES || code.lines().count() > MAX_HIGHLIGHT_LINES {
+    if code.len() > MAX_HIGHLIGHT_BYTES
+        || code.lines().count() > MAX_HIGHLIGHT_LINES
+        || code
+            .lines()
+            .any(|line| line.len() > MAX_HIGHLIGHT_LINE_BYTES)
+    {
         return None;
     }
 
@@ -1139,6 +1148,18 @@ mod tests {
         assert!(result.is_none(), "oversized input should fall back to None");
     }
 
+    #[test]
+    fn long_single_line_bash_skips_highlighting_and_preserves_text() {
+        let token = "eHh4".repeat(MAX_HIGHLIGHT_LINE_BYTES / 4 + 1);
+        let code = format!("printf %s {token} | base64 -d >/dev/null");
+
+        assert!(highlight_code_to_styled_spans(&code, "bash").is_none());
+        assert_eq!(
+            highlight_code_to_lines(&code, "bash"),
+            vec![Line::from(code)]
+        );
+    }
+
     #[test]
     fn highlight_many_lines_falls_back() {
         // Input exceeding MAX_HIGHLIGHT_LINES should return None.