fix(tui): add MAX_WRAP_INPUT_LEN guard to prevent UI freeze on oversized lines (closes #26)

- compute_line_entry/compute_visual_height: double guard (raw + post-format)
- Skip detect_level on oversized raw input to avoid O(n) JSON parsing
- Add post-format guard for JSON lines that expand beyond 10MB
- progressive_reader: add post-format guard to compute_line_visual_height
- Add truncate_to_columns helper using existing wrap_line_chars
- Fix misleading docstring on MAX_WRAP_INPUT_LEN constant
- Add 6 regression tests covering all guard paths

crates/core/src/io/progressive_reader.rs

@@ -223,6 +223,9 @@ pub fn compute_line_visual_height(
     }
     if json_format {
         let formatted = format_json_line(line_text);
+        if formatted.len() > MAX_WRAP_INPUT_LEN {
+            return 1;
+        }
         compute_text_visual_height(&formatted, terminal_width)
     } else {
         compute_text_visual_height(line_text, terminal_width)

crates/core/src/io/wrap.rs

@@ -1,5 +1,6 @@
 /// Maximum input length for wrap/format operations (10 MB).
-/// Lines exceeding this are returned as-is to avoid pathological cases.
+/// Callers should check against this constant before invoking `wrap_line_chars`
+/// to avoid pathological cases on oversized lines.
 pub const MAX_WRAP_INPUT_LEN: usize = 10 * 1024 * 1024;
 
 /// Split a line into chunks of exactly `width` display columns.

crates/tui/src/app.rs

@@ -6,7 +6,7 @@ use log_viewer_core::io::progressive_reader::{
     IndexerMessage, ProgressiveFileReader, VisualHeightIndex, compute_line_visual_height,
     spawn_indexer,
 };
-use log_viewer_core::io::wrap::{format_json_line, wrap_line_chars};
+use log_viewer_core::io::wrap::{format_json_line, wrap_line_chars, MAX_WRAP_INPUT_LEN};
 use log_viewer_core::types::LogLevel;
 use log_viewer_core::watcher::file_watcher::{FileEvent, FileWatcher};
 
@@ -201,12 +201,33 @@ impl App {
     /// Compute a single line's viewport entry (wrapped rows + level + height).
     fn compute_line_entry(&self, line: usize, width: usize) -> ViewportEntry {
         let raw = self.get_line(line).unwrap_or_default();
+
+        // Guard 1: oversized raw input — skip detect_level and JSON formatting
+        // to avoid O(n) parsing overhead on huge lines.
+        if raw.len() > MAX_WRAP_INPUT_LEN {
+            return ViewportEntry {
+                wrapped_rows: vec![truncate_to_columns(&raw, width)],
+                level: None,
+                visual_height: 1,
+            };
+        }
+
         let level = log_viewer_core::parser::level::detect_level(&raw);
         let display_text = if self.json_format {
             format_json_line(&raw)
         } else {
             raw
         };
+
+        // Guard 2: JSON pretty-printing may expand a line beyond the limit.
+        if display_text.len() > MAX_WRAP_INPUT_LEN {
+            return ViewportEntry {
+                wrapped_rows: vec![truncate_to_columns(&display_text, width)],
+                level,
+                visual_height: 1,
+            };
+        }
+
         let mut wrapped = Vec::new();
         for sub_line in display_text.split('\n') {
             wrapped.extend(wrap_line_chars(sub_line, width));
@@ -222,11 +243,23 @@ impl App {
     /// Compute visual height for a single line without storing it.
     fn compute_visual_height(&self, line: usize, width: usize) -> usize {
         let raw = self.get_line(line).unwrap_or_default();
+
+        // Guard 1: oversized raw input.
+        if raw.len() > MAX_WRAP_INPUT_LEN {
+            return 1;
+        }
+
         let display_text = if self.json_format {
             format_json_line(&raw)
         } else {
             raw
         };
+
+        // Guard 2: post-format expansion.
+        if display_text.len() > MAX_WRAP_INPUT_LEN {
+            return 1;
+        }
+
         let mut height = 0;
         for sub_line in display_text.split('\n') {
             height += wrap_line_chars(sub_line, width).len();
@@ -1154,6 +1187,13 @@ impl App {
     }
 }
 
+fn truncate_to_columns(s: &str, max_cols: usize) -> String {
+    if max_cols == 0 || s.is_empty() {
+        return String::new();
+    }
+    wrap_line_chars(s, max_cols).into_iter().next().unwrap_or_default()
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;
@@ -3419,4 +3459,93 @@ plain text line
         });
         assert!(result.is_ok());
     }
+
+    #[test]
+    fn test_compute_line_entry_oversized_raw_guard() {
+        let long_line = "x".repeat(MAX_WRAP_INPUT_LEN + 1);
+        let path = make_temp_file(&format!("short\n{}\n", long_line));
+        let result = std::panic::catch_unwind(|| {
+            let mut app = App::new();
+            load_file_ready(&mut app, &path);
+            assert!(app.is_loaded());
+
+            let entry = app.compute_line_entry(1, 80);
+            assert_eq!(entry.visual_height, 1, "oversized line should have height 1");
+            assert!(entry.level.is_none(), "oversized raw line should skip detect_level");
+            assert_eq!(entry.wrapped_rows.len(), 1);
+            assert!(
+                entry.wrapped_rows[0].len() <= 80,
+                "preview should be bounded to width"
+            );
+
+            let short_entry = app.compute_line_entry(0, 80);
+            assert_eq!(short_entry.visual_height, 1);
+            assert!(short_entry.level.is_none(), "'short' has no log level");
+        });
+        cleanup(&path);
+        assert!(result.is_ok());
+    }
+
+    #[test]
+    fn test_compute_visual_height_oversized_raw_guard() {
+        let long_line = "a".repeat(MAX_WRAP_INPUT_LEN + 100);
+        let path = make_temp_file(&format!("hi\n{}\n", long_line));
+        let result = std::panic::catch_unwind(|| {
+            let mut app = App::new();
+            load_file_ready(&mut app, &path);
+            assert!(app.is_loaded());
+
+            assert_eq!(app.compute_visual_height(0, 80), 1, "normal line height=1");
+            assert_eq!(app.compute_visual_height(1, 80), 1, "oversized line height=1");
+        });
+        cleanup(&path);
+        assert!(result.is_ok());
+    }
+
+    #[test]
+    fn test_compute_line_entry_post_format_guard() {
+        // Build a JSON object whose raw form is just under MAX_WRAP_INPUT_LEN
+        // but whose pretty-printed form (with added spaces around ':') exceeds it.
+        let inner = "x".repeat(MAX_WRAP_INPUT_LEN - 11);
+        let json_line = format!(r#"{{"msg":"{}"}}"#, inner);
+        assert!(
+            json_line.len() < MAX_WRAP_INPUT_LEN,
+            "raw JSON should be under limit: got {}",
+            json_line.len()
+        );
+
+        let path = make_temp_file(&format!("{}\n", json_line));
+        let result = std::panic::catch_unwind(|| {
+            let mut app = App::new();
+            load_file_ready(&mut app, &path);
+            app.json_format = true;
+
+            let entry = app.compute_line_entry(0, 80);
+            assert_eq!(entry.visual_height, 1, "post-format oversized should have height 1");
+            assert_eq!(entry.wrapped_rows.len(), 1);
+        });
+        cleanup(&path);
+        assert!(result.is_ok());
+    }
+
+    #[test]
+    fn test_truncate_to_columns_basic() {
+        assert_eq!(truncate_to_columns("hello world", 5), "hello");
+        assert_eq!(truncate_to_columns("hi", 80), "hi");
+        assert_eq!(truncate_to_columns("", 80), "");
+        assert_eq!(truncate_to_columns("abc", 0), "");
+    }
+
+    #[test]
+    fn test_truncate_to_columns_cjk() {
+        // Each CJK char is 2 columns wide
+        assert_eq!(truncate_to_columns("你好世界", 3), "你");
+        assert_eq!(truncate_to_columns("你好世界", 4), "你好");
+    }
+
+    #[test]
+    fn test_truncate_to_columns_tab() {
+        // Tab expands to 4 spaces
+        assert_eq!(truncate_to_columns("a\tb", 3), "a");
+    }
 }