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();
@@ -246,7 +279,6 @@ impl App {
     /// Returns (start_logical, offset_in_line) for rendering.
     pub(crate) fn ensure_viewport_cache(&mut self, width: usize) -> (usize, usize) {
         let viewport_height = self.content_height as usize;
-        let v_offset = self.v_offset;
 
         if !self.is_loaded() || width == 0 || viewport_height == 0 {
             return (0, 0);
@@ -268,7 +300,7 @@ impl App {
 
         // Find start logical line from v_offset
         let (start_logical, offset_in_line) = if self.is_loading() {
-            (v_offset.min(self.total_lines().saturating_sub(1)), self.v_sub_offset)
+            (self.v_offset.min(self.total_lines().saturating_sub(1)), self.v_sub_offset)
         } else {
             self.find_logical_line_at_visual_row(self.v_offset, width)
         };
@@ -660,12 +692,15 @@ impl App {
                 self.json_format = !self.json_format;
                 self.viewport_cache.invalidate();
                 let width = self.viewport_cache.width;
+                let (new_offset, new_sub) = self.rebase_offset_for_invalidate();
                 if let AppLoadingState::Ready { reader } = &mut self.loading_state {
                     reader.invalidate_visual_height_index();
                     if width > 0 {
                         reader.start_visual_height_rebuild(width, self.json_format);
                     }
                 }
+                self.v_offset = new_offset;
+                self.v_sub_offset = new_sub;
                 self.last_g_press = None;
             }
             KeyCode::Char('s') | KeyCode::Char('S')
@@ -824,6 +859,25 @@ impl App {
         }
     }
 
+    /// Compute the rebased offset pair (logical_line, sub_row) from the
+    /// current visual-row `v_offset`.  Returns `(v_offset, v_sub_offset)`
+    /// suitable for the no-VHI fallback scrolling path.
+    ///
+    /// MUST be called before borrowing `&mut self.loading_state` for
+    /// `invalidate_visual_height_index`, because it reads VHI through
+    /// `&self`.
+    fn rebase_offset_for_invalidate(&self) -> (usize, usize) {
+        if self.get_visual_height_index().is_some() {
+            let top_visual = self.v_offset;
+            let top_line = self.visual_row_to_logical_row(top_visual);
+            let line_first_visual = self.cursor_to_first_visual_row(top_line);
+            let sub = top_visual.saturating_sub(line_first_visual);
+            (top_line, sub)
+        } else {
+            (self.v_offset, self.v_sub_offset)
+        }
+    }
+
     fn ensure_visual_height_index(&mut self, width: usize) {
         let needs_rebuild = match self.get_visual_height_index() {
             Some(idx) => !idx.is_valid_for(self.json_format, width),
@@ -831,10 +885,13 @@ impl App {
         };
 
         if needs_rebuild {
+            let (new_offset, new_sub) = self.rebase_offset_for_invalidate();
             if let AppLoadingState::Ready { reader } = &mut self.loading_state {
                 reader.invalidate_visual_height_index();
                 reader.start_visual_height_rebuild(width, self.json_format);
             }
+            self.v_offset = new_offset;
+            self.v_sub_offset = new_sub;
         }
     }
 
@@ -917,6 +974,7 @@ impl App {
 
     fn handle_file_appended(&mut self) {
         let width = self.get_content_width();
+        let rebased = self.rebase_offset_for_invalidate();
         match &mut self.loading_state {
             AppLoadingState::Ready { reader } => {
                 let old_reader_line_count = reader.line_count();
@@ -967,6 +1025,9 @@ impl App {
                                 index.extend_from_heights(&new_heights);
                             }
                         } else {
+                            let (new_offset, new_sub) = rebased;
+                            self.v_offset = new_offset;
+                            self.v_sub_offset = new_sub;
                             reader.invalidate_visual_height_index();
                             reader.start_visual_height_rebuild(width, self.json_format);
                         }
@@ -975,9 +1036,11 @@ impl App {
                     }
                     Ok(log_viewer_core::io::file_reader::AppendStatus::Reloaded) => {
                         let _ = reader.save_cache();
+                        let (new_offset, _new_sub) = rebased;
                         reader.invalidate_visual_height_index();
                         reader.start_visual_height_rebuild(width, self.json_format);
                         self.cursor_line = self.cursor_line.min(self.total_lines().saturating_sub(1));
+                        self.v_offset = new_offset;
                         self.v_sub_offset = 0;
                         self.viewport_cache.invalidate();
                         self.clamp_v_offset();
@@ -994,12 +1057,14 @@ impl App {
 
     fn reload_ready_reader(&mut self) {
         let width = self.get_content_width();
+        let (new_offset, _new_sub) = self.rebase_offset_for_invalidate();
         if let AppLoadingState::Ready { reader } = &mut self.loading_state {
             let _ = reader.reload();
             let _ = reader.save_cache();
             reader.invalidate_visual_height_index();
             reader.start_visual_height_rebuild(width, self.json_format);
             self.cursor_line = self.cursor_line.min(self.total_lines().saturating_sub(1));
+            self.v_offset = new_offset;
             self.v_sub_offset = 0;
             self.viewport_cache.invalidate();
             self.clamp_v_offset();
@@ -1031,10 +1096,23 @@ impl App {
                 } = &mut reader.state
                 {
                     *visual_height_index = Some(index);
+                }
+            }
+        }
+        // Recalibrate v_offset from logical → visual-row now that VHI is available.
+        // Must be outside the `reader` borrow above.
+        if self.get_visual_height_index().is_some() {
+            let logical_top = self.v_offset.min(self.total_lines().saturating_sub(1));
+            let sub = self.v_sub_offset;
+            let first_visual = self.cursor_to_first_visual_row(logical_top);
+            let line_height = self
+                .get_visual_height_index()
+                .map_or(1, |idx| idx.visual_height_of_line(logical_top));
+            self.v_offset = first_visual.saturating_add(sub.min(line_height.saturating_sub(1)));
+            self.v_sub_offset = 0;
+            self.clamp_v_offset();
             self.viewport_cache.invalidate();
         }
-            }
-        }
 
         // Poll main indexer (Loading state)
         let old_state = std::mem::replace(&mut self.loading_state, AppLoadingState::Empty);
@@ -1079,9 +1157,12 @@ impl App {
 
                         // Gutter width changes (~N → N) shift content_width, so any
                         // VisualHeightIndex built with the old width is stale.
+                        let (new_offset, new_sub) = self.rebase_offset_for_invalidate();
                         if let AppLoadingState::Ready { reader } = &mut self.loading_state {
                             reader.invalidate_visual_height_index();
                         }
+                        self.v_offset = new_offset;
+                        self.v_sub_offset = new_sub;
 
                         if self.reload_after_loading {
                             self.reload_after_loading = false;
@@ -1106,6 +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::*;
@@ -2538,6 +2626,42 @@ plain text line
         assert!(result.is_ok());
     }
 
+    /// Regression test for issue #24:
+    /// ensure_viewport_cache must use the updated self.v_offset after params_changed
+    /// recalculates it, not a stale local captured before the change block.
+    #[test]
+    fn test_loading_viewport_cache_uses_updated_v_offset_on_params_changed() {
+        let content: String = (0..200).map(|i| format!("line{i}\n")).collect();
+        let path = make_temp_file(&content);
+        let result = std::panic::catch_unwind(|| {
+            let mut app = App::new();
+            app.load_file(path.to_str().unwrap()).unwrap();
+            assert!(app.is_loading(), "should be in Loading state");
+
+            app.content_height = 10;
+            app.ensure_viewport_cache(80);
+
+            app.v_offset = 90;
+            app.cursor_line = 100;
+
+            let new_width = 40;
+            app.ensure_viewport_cache(new_width);
+
+            let recomputed_offset = app.v_offset;
+            assert_ne!(
+                recomputed_offset, 90,
+                "v_offset should have been recalculated by params_changed block, still 90"
+            );
+
+            assert_eq!(
+                app.viewport_cache.logical_start, recomputed_offset.min(app.total_lines().saturating_sub(1)),
+                "logical_start should match the updated v_offset"
+            );
+        });
+        cleanup(&path);
+        assert!(result.is_ok());
+    }
+
     fn install_vhi(app: &mut App, heights: &[usize]) {
         let width = app.get_content_width();
         let json_format = app.json_format;
@@ -3091,4 +3215,337 @@ plain text line
         });
         assert!(result.is_ok());
     }
+
+    // ── Issue #25 regression tests ──────────────────────────────────
+    // Ready state without VHI must keep v_offset self-consistent.
+
+    #[test]
+    fn test_rebase_offset_converts_visual_to_logical_with_sub() {
+        let content = "line0\nline1\nline2\nline3\nline4\nline5\nline6\nline7\nline8\nline9\n";
+        let path = make_temp_file(content);
+        let result = std::panic::catch_unwind(|| {
+            let mut app = App::new();
+            load_file_ready(&mut app, &path);
+            app.content_height = 10;
+
+            // 10 lines × 3 visual rows each = 30 visual rows
+            install_vhi(&mut app, &[3usize; 10]);
+
+            // v_offset=7 → visual row 7 is inside line2 (rows 6-8), sub=1
+            app.v_offset = 7;
+            app.v_sub_offset = 0;
+
+            let (logical, sub) = app.rebase_offset_for_invalidate();
+            assert_eq!(logical, 2, "visual row 7 → logical line 2 (rows 6-8)");
+            assert_eq!(sub, 1, "visual row 7 is sub-row 1 within line 2");
+
+            // v_offset=0 → line 0, sub=0
+            app.v_offset = 0;
+            let (logical, sub) = app.rebase_offset_for_invalidate();
+            assert_eq!(logical, 0);
+            assert_eq!(sub, 0);
+
+            // v_offset=5 → line1 (rows 3-5), sub=2
+            app.v_offset = 5;
+            let (logical, sub) = app.rebase_offset_for_invalidate();
+            assert_eq!(logical, 1, "visual row 5 → logical line 1 (rows 3-5)");
+            assert_eq!(sub, 2);
+
+            cleanup(&path);
+        });
+        assert!(result.is_ok());
+    }
+
+    #[test]
+    fn test_rebase_offset_passthrough_when_no_vhi() {
+        let content = "line0\nline1\nline2\n";
+        let path = make_temp_file(content);
+        let result = std::panic::catch_unwind(|| {
+            let mut app = App::new();
+            load_file_ready(&mut app, &path);
+
+            // No VHI installed — rebase should return current values unchanged
+            app.v_offset = 42;
+            app.v_sub_offset = 3;
+            let (logical, sub) = app.rebase_offset_for_invalidate();
+            assert_eq!(logical, 42);
+            assert_eq!(sub, 3);
+
+            cleanup(&path);
+        });
+        assert!(result.is_ok());
+    }
+
+    #[test]
+    fn test_tab_toggle_rebases_offset_before_invalidate() {
+        let content = "line0\nline1\nline2\nline3\nline4\nline5\nline6\nline7\nline8\nline9\n";
+        let path = make_temp_file(content);
+        let result = std::panic::catch_unwind(|| {
+            let mut app = App::new();
+            load_file_ready(&mut app, &path);
+            app.content_height = 10;
+            app.content_width = 20;
+
+            install_vhi(&mut app, &[3usize; 10]);
+
+            // Scroll to visual row 7 → line2 sub-row 1
+            app.v_offset = 7;
+            app.cursor_line = 2;
+            app.v_sub_offset = 0;
+
+            // Simulate Tab toggle: rebase, then invalidate
+            let rebased = app.rebase_offset_for_invalidate();
+            let (new_offset, new_sub) = rebased;
+            app.v_offset = new_offset;
+            app.v_sub_offset = new_sub;
+
+            assert_eq!(app.v_offset, 2, "v_offset should be logical line 2 after rebase");
+            assert_eq!(app.v_sub_offset, 1, "v_sub_offset should preserve sub-row 1");
+
+            // Key invariant: v_offset is now a valid logical line number,
+            // not a stale visual-row offset that would cause a jump.
+            // Before the fix, v_offset would still be 7 (visual) treated as logical → jump.
+            assert!(
+                app.v_offset < app.total_lines(),
+                "v_offset ({}) must be a valid logical line index < {}",
+                app.v_offset, app.total_lines()
+            );
+
+            cleanup(&path);
+        });
+        assert!(result.is_ok());
+    }
+
+    #[test]
+    fn test_vhi_rebuild_recalibrates_from_viewport_top() {
+        let content = "line0\nline1\nline2\nline3\nline4\nline5\nline6\nline7\nline8\nline9\n";
+        let path = make_temp_file(content);
+        let result = std::panic::catch_unwind(|| {
+            let mut app = App::new();
+            load_file_ready(&mut app, &path);
+            app.content_height = 10;
+            app.content_width = 20;
+
+            // Install VHI with varying heights
+            install_vhi(&mut app, &[2, 4, 3, 1, 2, 5, 1, 3, 2, 1]);
+
+            // Scroll to visual row 8 → line2 (rows 6-8), sub=2
+            app.v_offset = 8;
+            app.cursor_line = 2;
+            app.v_sub_offset = 0;
+
+            // Rebase (simulates invalidate)
+            let (logical_top, sub) = app.rebase_offset_for_invalidate();
+            assert_eq!(logical_top, 2);
+            assert_eq!(sub, 2);
+
+            // Simulate what happens: v_offset becomes logical, VHI cleared
+            app.v_offset = logical_top;
+            app.v_sub_offset = sub;
+
+            // Now simulate VHI rebuild completion — the recalibration block
+            // Install a fresh VHI (same heights, simulates rebuild result)
+            install_vhi(&mut app, &[2, 4, 3, 1, 2, 5, 1, 3, 2, 1]);
+
+            // The recalibration logic from poll_background_indexer
+            let logical_top = app.v_offset.min(app.total_lines().saturating_sub(1));
+            let sub = app.v_sub_offset;
+            let first_visual = app.cursor_to_first_visual_row(logical_top);
+            let line_height = app
+                .get_visual_height_index()
+                .map_or(1, |idx| idx.visual_height_of_line(logical_top));
+            app.v_offset = first_visual.saturating_add(sub.min(line_height.saturating_sub(1)));
+            app.v_sub_offset = 0;
+
+            // line2 starts at visual row 6, sub was 2 → visual row 8
+            assert_eq!(app.v_offset, 8, "v_offset should map back to visual row 8");
+            assert_eq!(app.v_sub_offset, 0, "v_sub_offset should be 0 after recalibration");
+
+            // Scrolling should work normally with VHI
+            app.scroll_down_line();
+            assert_eq!(app.v_offset, 9, "scroll down should advance visual row");
+
+            cleanup(&path);
+        });
+        assert!(result.is_ok());
+    }
+
+    #[test]
+    fn test_vhi_rebuild_clamps_sub_to_new_line_height() {
+        let content = "line0\nline1\nline2\n";
+        let path = make_temp_file(content);
+        let result = std::panic::catch_unwind(|| {
+            let mut app = App::new();
+            load_file_ready(&mut app, &path);
+            app.content_height = 24;
+            app.content_width = 20;
+
+            // Line heights: line0=1, line1=5 (wraps), line2=1
+            install_vhi(&mut app, &[1, 5, 1]);
+
+            // Position at line1 sub-row 4 (last sub-row of 5-row line)
+            app.v_offset = 5;
+            app.cursor_line = 1;
+            app.v_sub_offset = 0;
+
+            // Rebase
+            let (logical_top, sub) = app.rebase_offset_for_invalidate();
+            assert_eq!(logical_top, 1, "visual row 5 → line 1 (rows 1-5)");
+            assert_eq!(sub, 4, "sub-row 4 within line 1");
+
+            app.v_offset = logical_top;
+            app.v_sub_offset = sub;
+
+            // Simulate VHI rebuild with SHRUNK line height (e.g., JSON toggle)
+            // line1 now only 2 visual rows instead of 5
+            install_vhi(&mut app, &[1, 2, 1]);
+
+            // Recalibration should clamp sub=4 to new height-1=1
+            let logical_top = app.v_offset.min(app.total_lines().saturating_sub(1));
+            let sub = app.v_sub_offset;
+            let first_visual = app.cursor_to_first_visual_row(logical_top);
+            let line_height = app
+                .get_visual_height_index()
+                .map_or(1, |idx| idx.visual_height_of_line(logical_top));
+            let clamped_sub = sub.min(line_height.saturating_sub(1));
+            app.v_offset = first_visual.saturating_add(clamped_sub);
+            app.v_sub_offset = 0;
+
+            // line1 starts at visual row 1, clamped sub=1 → visual row 2
+            assert_eq!(app.v_offset, 2, "v_offset should be clamped to visual row 2 (line1 row 1 of 2)");
+            assert_eq!(app.v_sub_offset, 0);
+
+            cleanup(&path);
+        });
+        assert!(result.is_ok());
+    }
+
+    #[test]
+    fn test_loading_to_ready_rebases_visual_offset() {
+        let content = "line0\nline1\nline2\nline3\nline4\n";
+        let path = make_temp_file(content);
+        let result = std::panic::catch_unwind(|| {
+            let mut app = App::new();
+            load_file_ready(&mut app, &path);
+            app.content_height = 10;
+
+            // Install VHI: 2 rows per line
+            install_vhi(&mut app, &[2usize; 5]);
+
+            // Scroll to visual row 3 → line1 sub-row 1
+            app.v_offset = 3;
+            app.cursor_line = 1;
+            app.v_sub_offset = 0;
+
+            // Simulate Loading→Ready invalidation rebase
+            let (new_offset, new_sub) = app.rebase_offset_for_invalidate();
+            assert_eq!(new_offset, 1, "visual row 3 → line 1");
+            assert_eq!(new_sub, 1, "sub-row 1 within line 1");
+
+            // Apply rebased values and clear VHI (simulate invalidate)
+            app.v_offset = new_offset;
+            app.v_sub_offset = new_sub;
+            if let AppLoadingState::Ready { reader } = &mut app.loading_state {
+                reader.invalidate_visual_height_index();
+            }
+
+            // VHI is now None → scroll_down_line uses else branch (v_sub_offset path)
+            // "line1" at width=80 → compute_visual_height=1, sub=1+1 >= 1 → advance
+            app.scroll_down_line();
+            assert_eq!(app.v_offset, 2, "should advance to line 2 (line1 height=1, sub overflow)");
+            assert_eq!(app.v_sub_offset, 0);
+
+            cleanup(&path);
+        });
+        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");
+    }
 }