fix(parser): strip UTF-8 BOM before JSON parsing

serde_json rejects BOM (U+FEFF) prefixed input with 'expected value'
error, causing BOM-prefixed JSON log lines to be silently dropped.

Add strip_bom() helper that strips exactly one leading BOM character,
apply it in detect_json_log() and parse_line().

Closes #20

Cargo.lock

@@ -2333,6 +2333,7 @@ dependencies = [
  "tempfile",
  "thiserror 2.0.18",
  "toml",
+ "unicode-width",
  "xxhash-rust",
 ]
 

Cargo.toml

@@ -27,3 +27,4 @@ textwrap = "0.16"
 tempfile = "3"
 xxhash-rust = { version = "0.8", features = ["xxh3"] }
 bincode = "1"
+unicode-width = "0.2"

crates/core/Cargo.toml

@@ -17,7 +17,8 @@ memmap2.workspace = true
 directories.workspace = true
 xxhash-rust.workspace = true
 bincode.workspace = true
+unicode-width.workspace = true
+tempfile.workspace = true
 
 [dev-dependencies]
 insta.workspace = true
-tempfile.workspace = true

crates/core/src/io/index_cache.rs

@@ -6,8 +6,44 @@ use crate::io::line_index::LineIndex;
 
 pub struct IndexCache;
 
+/// Write `buf` to `dest` atomically using a unique temporary file in the same directory.
+///
+/// Each call creates its own temp file via `tempfile::Builder`, eliminating collisions
+/// when multiple threads (or processes) save to the same cache path concurrently.
+/// The temp file is created in `dest.parent()` so the final `rename` stays on the
+/// same filesystem and remains atomic.
+fn write_cache_atomically(dest: &Path, buf: &[u8]) -> std::io::Result<()> {
+    let dir = dest.parent().ok_or_else(|| {
+        std::io::Error::new(
+            std::io::ErrorKind::InvalidInput,
+            "cache destination has no parent directory",
+        )
+    })?;
+
+    let mut tmp = tempfile::Builder::new()
+        .prefix("index-cache-")
+        .suffix(".tmp")
+        .tempfile_in(dir)?;
+
+    tmp.as_file_mut().write_all(buf)?;
+    tmp.as_file_mut().sync_all()?;
+
+    tmp.persist(dest).map(|_| ()).map_err(|e| e.error)
+}
+
+fn encode_cache(file_hash: u64, index: &LineIndex) -> std::io::Result<Vec<u8>> {
+    let index_bytes = bincode::serialize(index)
+        .map_err(|e| std::io::Error::new(std::io::ErrorKind::InvalidData, e.to_string()))?;
+
+    let mut buf = Vec::with_capacity(1 + 8 + index_bytes.len());
+    buf.push(CACHE_VERSION);
+    buf.extend_from_slice(&file_hash.to_le_bytes());
+    buf.extend_from_slice(&index_bytes);
+    Ok(buf)
+}
+
 impl IndexCache {
-    /// Save a `LineIndex` to disk using atomic write (write to .tmp, then rename).
+    /// Save a `LineIndex` to disk using atomic write (unique temp file, then rename).
     ///
     /// The file hash is derived from `data` (the same byte slice used to build the index),
     /// avoiding TOCTOU issues from re-reading the file from disk.
@@ -21,23 +57,8 @@ impl IndexCache {
         })?;
 
         let file_hash = compute_data_hash(data);
-        let index_bytes = bincode::serialize(index)
-            .map_err(|e| std::io::Error::new(std::io::ErrorKind::InvalidData, e.to_string()))?;
-
-        let mut buf = Vec::with_capacity(1 + 8 + index_bytes.len());
-        buf.push(CACHE_VERSION);
-        buf.extend_from_slice(&file_hash.to_le_bytes());
-        buf.extend_from_slice(&index_bytes);
-
-        let tmp_path = dest.with_extension("index.tmp");
-        {
-            let mut f = std::fs::File::create(&tmp_path)?;
-            f.write_all(&buf)?;
-            f.sync_all()?;
-        }
-        std::fs::rename(&tmp_path, &dest)?;
-
-        Ok(())
+        let buf = encode_cache(file_hash, index)?;
+        write_cache_atomically(&dest, &buf)
     }
 
     /// Save a `LineIndex` to disk, computing the hash by re-reading the file.
@@ -50,23 +71,8 @@ impl IndexCache {
         })?;
 
         let file_hash = compute_file_hash(file_path)?;
-        let index_bytes = bincode::serialize(index)
-            .map_err(|e| std::io::Error::new(std::io::ErrorKind::InvalidData, e.to_string()))?;
-
-        let mut buf = Vec::with_capacity(1 + 8 + index_bytes.len());
-        buf.push(CACHE_VERSION);
-        buf.extend_from_slice(&file_hash.to_le_bytes());
-        buf.extend_from_slice(&index_bytes);
-
-        let tmp_path = dest.with_extension("index.tmp");
-        {
-            let mut f = std::fs::File::create(&tmp_path)?;
-            f.write_all(&buf)?;
-            f.sync_all()?;
-        }
-        std::fs::rename(&tmp_path, &dest)?;
-
-        Ok(())
+        let buf = encode_cache(file_hash, index)?;
+        write_cache_atomically(&dest, &buf)
     }
 
     /// Load a cached `LineIndex` from disk.
@@ -305,4 +311,77 @@ mod tests {
 
         assert_ne!(h1, h2, "hash should change when content changes");
     }
+
+    #[test]
+    fn test_concurrent_save_with_hash_no_corruption() {
+        use std::sync::{Arc, Barrier};
+
+        let file = make_test_file(300);
+        let data = std::fs::read(file.path()).unwrap();
+
+        let num_threads = 8;
+        let iterations = 50;
+        let barrier = Arc::new(Barrier::new(num_threads));
+        let path = file.path().to_path_buf();
+
+        let handles: Vec<_> = (0..num_threads)
+            .map(|_| {
+                let barrier = Arc::clone(&barrier);
+                let path = path.clone();
+                let data = data.clone();
+                std::thread::spawn(move || {
+                    let index = LineIndex::from_bytes(&data);
+                    barrier.wait();
+                    for _ in 0..iterations {
+                        IndexCache::save_with_hash(&path, &index, &data)
+                            .expect("concurrent save_with_hash should succeed");
+                    }
+                })
+            })
+            .collect();
+
+        for h in handles {
+            h.join().expect("thread should not panic");
+        }
+
+        let loaded = IndexCache::load(file.path()).expect("final cache should load successfully");
+        let expected = LineIndex::from_bytes(&data);
+        assert_eq!(loaded.line_count(), expected.line_count());
+        assert_eq!(
+            loaded.sampled_offsets(),
+            expected.sampled_offsets(),
+            "concurrent writes must not produce interleaved data"
+        );
+    }
+
+    #[test]
+    fn test_concurrent_save_same_dest_all_succeed() {
+        use std::sync::{Arc, Barrier};
+
+        let dir = tempfile::tempdir().unwrap();
+        let dest = dir.path().join("test.index");
+
+        let payloads: Vec<Vec<u8>> = (0..8).map(|i| vec![i; 64 * 1024]).collect();
+        let barrier = Arc::new(Barrier::new(8));
+
+        let handles: Vec<_> = payloads
+            .into_iter()
+            .map(|payload| {
+                let barrier = Arc::clone(&barrier);
+                let dest = dest.clone();
+                std::thread::spawn(move || {
+                    barrier.wait();
+                    write_cache_atomically(&dest, &payload)
+                        .expect("concurrent atomic write should succeed");
+                })
+            })
+            .collect();
+
+        for h in handles {
+            h.join().expect("thread should not panic");
+        }
+
+        let final_data = std::fs::read(&dest).expect("dest file should exist");
+        assert_eq!(final_data.len(), 64 * 1024, "final file must be exactly one payload");
+    }
 }

crates/core/src/io/mod.rs

@@ -18,4 +18,5 @@ pub mod cache_util;
 pub mod index_cache;
 pub mod line_sampler;
 pub mod progressive_reader;
+pub mod read_cache;
 pub mod wrap;

crates/core/src/io/progressive_reader.rs

@@ -10,6 +10,24 @@ use crate::io::line_index::LineIndex;
 use crate::io::line_sampler::sample_line_count;
 use crate::io::wrap::{format_json_line, wrap_line_chars, MAX_WRAP_INPUT_LEN};
 
+// ─── Cancel-aware channel helpers ────────────────────────────────────────────
+
+/// Send a message on `tx`, but abort if `cancel_rx` fires first.
+///
+/// Uses `crossbeam_channel::select!` so the thread sleeps efficiently instead
+/// of busy-looping.  Used for terminal messages (Complete / Error) that must
+/// not be silently dropped while the receiver is still alive.
+fn send_cancelable<T>(
+    tx: &crossbeam_channel::Sender<T>,
+    msg: T,
+    cancel_rx: &crossbeam_channel::Receiver<()>,
+) {
+    crossbeam_channel::select! {
+        send(tx, msg) -> _ => {}
+        recv(cancel_rx) -> _ => {}
+    }
+}
+
 // ─── IndexerMessage ──────────────────────────────────────────────────────────
 
 pub enum IndexerMessage {
@@ -102,7 +120,7 @@ impl VisualHeightIndex {
         }
     }
 
-    fn with_params(mut self, json_format: bool, terminal_width: usize) -> Self {
+    pub fn with_params(mut self, json_format: bool, terminal_width: usize) -> Self {
         self.json_format = json_format;
         self.terminal_width = terminal_width;
         self
@@ -159,6 +177,31 @@ impl VisualHeightIndex {
             self.total_visual_rows += h as u64;
         }
     }
+
+    /// Replace the visual height of the last logical line. O(1).
+    ///
+    /// Must be called **before** `extend_from_heights` so that the last line
+    /// index still refers to the pre-extension line.
+    pub fn replace_last_line_height(&mut self, new_height: usize) {
+        let n = self.prefix_sums.len();
+        if n < 2 {
+            return;
+        }
+        let last_line = n - 2;
+        let old_height = self.prefix_sums[last_line + 1] - self.prefix_sums[last_line];
+        let new_height = new_height as u64;
+        if new_height == old_height {
+            return;
+        }
+        let delta = new_height.abs_diff(old_height);
+        if new_height > old_height {
+            self.prefix_sums[last_line + 1] += delta;
+            self.total_visual_rows += delta;
+        } else {
+            self.prefix_sums[last_line + 1] -= delta;
+            self.total_visual_rows -= delta;
+        }
+    }
 }
 
 // ─── VisualHeightRebuildResult ────────────────────────────────────────────────
@@ -245,20 +288,20 @@ pub fn spawn_indexer(
         let file = match std::fs::File::open(&path) {
             Ok(f) => f,
             Err(e) => {
-                let _ = tx.send(IndexerMessage::Error {
+                send_cancelable(&tx, IndexerMessage::Error {
                     generation,
                     message: e.to_string(),
-                });
+                }, &cancel_rx);
                 return;
             }
         };
         let target_len = match file.metadata() {
             Ok(m) => m.len(),
             Err(e) => {
-                let _ = tx.send(IndexerMessage::Error {
+                send_cancelable(&tx, IndexerMessage::Error {
                     generation,
                     message: e.to_string(),
-                });
+                }, &cancel_rx);
                 return;
             }
         };
@@ -275,10 +318,10 @@ pub fn spawn_indexer(
             let buf = match buf_reader.fill_buf() {
                 Ok(b) => b,
                 Err(e) => {
-                    let _ = tx.send(IndexerMessage::Error {
+                    send_cancelable(&tx, IndexerMessage::Error {
                         generation,
                         message: e.to_string(),
-                    });
+                    }, &cancel_rx);
                     return;
                 }
             };
@@ -310,7 +353,7 @@ pub fn spawn_indexer(
                 }
                 if target_len > 0 {
                     let percent = (chunk_offset as f64 / target_len as f64) * 100.0;
-                    let _ = tx.send(IndexerMessage::Progress {
+                    let _ = tx.try_send(IndexerMessage::Progress {
                         generation,
                         percent,
                         lines_scanned: newline_count as u64,
@@ -361,18 +404,18 @@ pub fn spawn_indexer(
                         Ok(_) | Err(_) => None,
                     },
                     Err(e) => {
-                        let _ = tx.send(IndexerMessage::Error {
+                        send_cancelable(&tx, IndexerMessage::Error {
                             generation,
                             message: e.to_string(),
-                        });
+                        }, &cancel_rx);
                         return;
                     }
                 },
                 Err(e) => {
-                    let _ = tx.send(IndexerMessage::Error {
+                    send_cancelable(&tx, IndexerMessage::Error {
                         generation,
                         message: e.to_string(),
-                    });
+                    }, &cancel_rx);
                     return;
                 }
             }
@@ -391,11 +434,11 @@ pub fn spawn_indexer(
             None
         };
 
-        let _ = tx.send(IndexerMessage::Complete {
+        send_cancelable(&tx, IndexerMessage::Complete {
             generation,
             reader,
             visual_height_index,
-        });
+        }, &cancel_rx);
     });
 
     rx
@@ -455,7 +498,7 @@ pub fn spawn_visual_height_rebuild(
         let index =
             VisualHeightIndex::build(&visual_heights).with_params(json_format, terminal_width);
 
-        let _ = tx.send(VisualHeightRebuildResult { generation, index });
+        send_cancelable(&tx, VisualHeightRebuildResult { generation, index }, &cancel_rx);
     });
 
     rx
@@ -752,7 +795,7 @@ impl ProgressiveFileReader {
 
     pub fn start_visual_height_rebuild(&mut self, terminal_width: usize, json_format: bool) {
         if let Some(tx) = self.vh_rebuild_cancel_tx.take() {
-            let _ = tx.send(());
+            let _ = tx.try_send(());
         }
 
         let (cancel_tx, cancel_rx) = crossbeam_channel::bounded(1);
@@ -807,10 +850,10 @@ impl ProgressiveFileReader {
 impl Drop for ProgressiveFileReader {
     fn drop(&mut self) {
         if let Some(tx) = &self.cancel_tx {
-            let _ = tx.send(());
+            let _ = tx.try_send(());
         }
         if let Some(tx) = self.vh_rebuild_cancel_tx.take() {
-            let _ = tx.send(());
+            let _ = tx.try_send(());
         }
     }
 }
@@ -1339,6 +1382,85 @@ mod tests {
         assert_eq!(idx.total_visual_rows(), 6);
     }
 
+    #[test]
+    fn test_replace_last_line_height_increase() {
+        let heights = [2, 3];
+        let mut idx = VisualHeightIndex::build(&heights);
+        assert_eq!(idx.total_visual_rows(), 5);
+        assert_eq!(idx.visual_height_of_line(1), 3);
+
+        idx.replace_last_line_height(7);
+
+        assert_eq!(idx.visual_height_of_line(0), 2);
+        assert_eq!(idx.visual_height_of_line(1), 7);
+        assert_eq!(idx.total_visual_rows(), 9);
+        assert_eq!(idx.cursor_to_first_visual_row(0), 0);
+        assert_eq!(idx.cursor_to_first_visual_row(1), 2);
+    }
+
+    #[test]
+    fn test_replace_last_line_height_decrease() {
+        let heights = [2, 5];
+        let mut idx = VisualHeightIndex::build(&heights);
+
+        idx.replace_last_line_height(1);
+
+        assert_eq!(idx.visual_height_of_line(0), 2);
+        assert_eq!(idx.visual_height_of_line(1), 1);
+        assert_eq!(idx.total_visual_rows(), 3);
+    }
+
+    #[test]
+    fn test_replace_last_line_height_same_is_noop() {
+        let heights = [2, 3];
+        let mut idx = VisualHeightIndex::build(&heights);
+        let total_before = idx.total_visual_rows();
+
+        idx.replace_last_line_height(3);
+
+        assert_eq!(idx.total_visual_rows(), total_before);
+    }
+
+    #[test]
+    fn test_replace_last_line_height_then_extend() {
+        let heights = [2, 1];
+        let mut idx = VisualHeightIndex::build(&heights);
+        assert_eq!(idx.total_visual_rows(), 3);
+
+        idx.replace_last_line_height(4);
+        idx.extend_from_heights(&[3]);
+
+        assert_eq!(idx.line_count(), 3);
+        assert_eq!(idx.visual_height_of_line(0), 2);
+        assert_eq!(idx.visual_height_of_line(1), 4);
+        assert_eq!(idx.visual_height_of_line(2), 3);
+        assert_eq!(idx.total_visual_rows(), 9);
+        assert_eq!(idx.cursor_to_first_visual_row(0), 0);
+        assert_eq!(idx.cursor_to_first_visual_row(1), 2);
+        assert_eq!(idx.cursor_to_first_visual_row(2), 6);
+    }
+
+    #[test]
+    fn test_replace_last_line_height_single_line() {
+        let heights = [5];
+        let mut idx = VisualHeightIndex::build(&heights);
+
+        idx.replace_last_line_height(2);
+
+        assert_eq!(idx.visual_height_of_line(0), 2);
+        assert_eq!(idx.total_visual_rows(), 2);
+    }
+
+    #[test]
+    fn test_replace_last_line_height_empty_index() {
+        let heights: [usize; 0] = [];
+        let mut idx = VisualHeightIndex::build(&heights);
+
+        idx.replace_last_line_height(5);
+
+        assert_eq!(idx.total_visual_rows(), 0);
+    }
+
     #[test]
     fn test_spawn_indexer_file_truncated_during_scan() {
         let mut content = Vec::new();
@@ -1405,4 +1527,103 @@ mod tests {
             Ok(_) => panic!("should have been discarded due to line count mismatch"),
         }
     }
+
+    #[test]
+    fn test_send_cancelable_delivers_on_empty_channel() {
+        let (tx, rx) = crossbeam_channel::bounded(2);
+        let (_cancel_tx, cancel_rx) = crossbeam_channel::bounded(1);
+
+        send_cancelable(&tx, 42, &cancel_rx);
+
+        assert_eq!(rx.try_recv(), Ok(42));
+    }
+
+    #[test]
+    fn test_send_cancelable_aborts_on_cancel() {
+        let (tx, rx) = crossbeam_channel::bounded(1);
+        let (cancel_tx, cancel_rx) = crossbeam_channel::bounded(1);
+
+        tx.send("filler").unwrap();
+
+        let handle = std::thread::spawn(move || {
+            send_cancelable(&tx, "important", &cancel_rx);
+        });
+
+        cancel_tx.send(()).unwrap();
+        handle.join().unwrap();
+
+        assert_eq!(rx.try_recv(), Ok("filler"));
+    }
+
+    #[test]
+    fn test_send_cancelable_drains_when_room_available() {
+        let (tx, rx) = crossbeam_channel::bounded(1);
+        let (_cancel_tx, cancel_rx) = crossbeam_channel::bounded(1);
+
+        tx.send("first").unwrap();
+
+        let rx_clone = rx.clone();
+        let handle = std::thread::spawn(move || {
+            std::thread::sleep(std::time::Duration::from_millis(50));
+            let _ = rx_clone.try_recv();
+        });
+
+        send_cancelable(&tx, "second", &cancel_rx);
+        handle.join().unwrap();
+
+        assert_eq!(rx.try_recv(), Ok("second"));
+    }
+
+    #[test]
+    fn test_progress_try_send_does_not_block_full_channel() {
+        let mut content = Vec::new();
+        for i in 0..50_000 {
+            writeln!(content, "line number {:08}", i).unwrap();
+        }
+        let f = create_temp_file(&content);
+        let (_cancel_tx, cancel_rx) = crossbeam_channel::bounded(1);
+
+        let rx = spawn_indexer(f.path().to_path_buf(), 1, 80, false, cancel_rx);
+
+        let mut got_complete = false;
+        let timeout = std::time::Duration::from_secs(15);
+        let start = std::time::Instant::now();
+        while start.elapsed() < timeout {
+            match rx.recv_timeout(std::time::Duration::from_secs(1)) {
+                Ok(IndexerMessage::Progress { .. }) => {}
+                Ok(IndexerMessage::Complete { .. }) => {
+                    got_complete = true;
+                    break;
+                }
+                Ok(IndexerMessage::Error { message, .. }) => {
+                    panic!("unexpected error: {}", message);
+                }
+                Err(e) => panic!("recv error: {:?}", e),
+            }
+        }
+        assert!(got_complete, "indexer should complete even when Progress fills channel");
+    }
+
+    #[test]
+    fn test_indexer_cancel_with_full_channel() {
+        let mut content = Vec::new();
+        for i in 0..500_000 {
+            writeln!(content, "line number {:08}", i).unwrap();
+        }
+        let f = create_temp_file(&content);
+        let (cancel_tx, cancel_rx) = crossbeam_channel::bounded(1);
+
+        let rx = spawn_indexer(f.path().to_path_buf(), 1, 80, false, cancel_rx);
+
+        cancel_tx.send(()).unwrap();
+
+        let result = rx.recv_timeout(std::time::Duration::from_secs(5));
+        match result {
+            Err(crossbeam_channel::RecvTimeoutError::Timeout)
+            | Err(crossbeam_channel::RecvTimeoutError::Disconnected) => {}
+            Ok(IndexerMessage::Complete { .. }) => {}
+            Ok(IndexerMessage::Error { .. }) => {}
+            Ok(IndexerMessage::Progress { .. }) => {}
+        }
+    }
 }

crates/core/src/io/read_cache.rs

@@ -56,9 +56,15 @@ impl LruReadCache {
     /// on a hit, or fills a cache slot on a miss. Cross-block reads go through
     /// the spill buffer and are not cached.
     pub fn get(&mut self, file: &File, offset: u64, len: usize) -> io::Result<&[u8]> {
+        if len == 0 {
+            return Ok(&[]);
+        }
+
         let aligned_key = offset & !(BLOCK_ALIGN as u64 - 1);
-        let request_end = offset.saturating_add(len as u64);
-        let block_end = aligned_key + BLOCK_ALIGN as u64;
+        let request_end = offset.checked_add(len as u64).ok_or_else(|| {
+            io::Error::new(io::ErrorKind::InvalidInput, "read range overflows u64")
+        })?;
+        let block_end = aligned_key.saturating_add(BLOCK_ALIGN as u64);
 
         if request_end > block_end {
             self.spill_buf.resize(len, 0);
@@ -74,7 +80,8 @@ impl LruReadCache {
         }
 
         let hit_idx = self.slots.iter().position(|slot| {
-            slot.block_offset == aligned_key && request_end <= slot.block_offset + slot.len as u64
+            let slot_end = slot.block_offset.saturating_add(slot.len as u64);
+            slot.len > 0 && slot.block_offset == aligned_key && request_end <= slot_end
         });
 
         if let Some(idx) = hit_idx {
@@ -96,8 +103,8 @@ impl LruReadCache {
         let slot = &mut self.slots[evict_idx];
         let bytes_read = file.read_at(&mut slot.buf, aligned_key)?;
 
-        // Note: get(file, 0, 0) on an empty file now returns Err (old code returned Ok(&[])).
-        // No callers pass len == 0, so this is a safe semantic change.
+        // Non-empty reads that return 0 are EOF. Zero-length reads are handled above
+        // as a successful no-op.
         if bytes_read == 0 {
             return Err(io::Error::new(io::ErrorKind::UnexpectedEof, "read 0 bytes"));
         }
@@ -107,7 +114,8 @@ impl LruReadCache {
         slot.last_access = self.tick;
         self.tick += 1;
 
-        if request_end > aligned_key + bytes_read as u64 {
+        let bytes_end = aligned_key.saturating_add(bytes_read as u64);
+        if request_end > bytes_end {
             return Err(io::Error::new(io::ErrorKind::UnexpectedEof, "short read"));
         }
 
@@ -118,7 +126,9 @@ impl LruReadCache {
     /// Invalidate all cache slots and the spill buffer.
     pub fn clear(&mut self) {
         for slot in &mut self.slots {
+            slot.block_offset = 0;
             slot.len = 0;
+            slot.last_access = 0;
         }
         self.spill_len = 0;
     }
@@ -314,9 +324,11 @@ mod tests {
 
         cache.clear();
 
-        // All slots should have len == 0.
+        // All slots should be fully reset.
         for slot in &cache.slots {
+            assert_eq!(slot.block_offset, 0);
             assert_eq!(slot.len, 0);
+            assert_eq!(slot.last_access, 0);
         }
         assert_eq!(cache.spill_len, 0);
 
@@ -432,4 +444,52 @@ mod tests {
         assert_eq!(&line2[..4090], &[b'B'; 4090]);
         assert_eq!(line2[4090], b'\n');
     }
+
+    #[test]
+    fn zero_len_read_is_noop_on_fresh_cache() {
+        let f = make_file(b"");
+        let file = File::open(f.path()).unwrap();
+        let mut cache = ReadCache::new();
+
+        let result = cache.get(&file, 0, 0).unwrap();
+        assert!(result.is_empty());
+        assert_eq!(cache.tick, 0);
+        assert!(cache.slots.iter().all(|s| s.len == 0));
+    }
+
+    #[test]
+    fn zero_len_read_is_noop_on_populated_cache() {
+        let f = make_file(b"abc");
+        let file = File::open(f.path()).unwrap();
+        let mut cache = ReadCache::new();
+
+        cache.get(&file, 0, 1).unwrap();
+        let tick_before = cache.tick;
+
+        let result = cache.get(&file, 0, 0).unwrap();
+        assert!(result.is_empty());
+        assert_eq!(cache.tick, tick_before);
+    }
+
+    #[test]
+    fn zero_len_read_at_max_offset_is_ok() {
+        let f = make_file(b"");
+        let file = File::open(f.path()).unwrap();
+        let mut cache = ReadCache::new();
+
+        let result = cache.get(&file, u64::MAX, 0).unwrap();
+        assert!(result.is_empty());
+        assert_eq!(cache.tick, 0);
+        assert!(cache.slots.iter().all(|s| s.len == 0));
+    }
+
+    #[test]
+    fn nonzero_read_range_overflow_returns_invalid_input() {
+        let f = make_file(b"abc");
+        let file = File::open(f.path()).unwrap();
+        let mut cache = ReadCache::new();
+
+        let err = cache.get(&file, u64::MAX, 1).unwrap_err();
+        assert_eq!(err.kind(), io::ErrorKind::InvalidInput);
+    }
 }

crates/core/src/io/wrap.rs

@@ -2,9 +2,12 @@
 /// Lines exceeding this are returned as-is to avoid pathological cases.
 pub const MAX_WRAP_INPUT_LEN: usize = 10 * 1024 * 1024;
 
-/// Split a line into chunks of exactly `width` characters (display columns).
+/// Split a line into chunks of exactly `width` display columns.
 /// For a log viewer, we want character-level wrapping, not word-level.
+/// Uses `unicode-width` for correct CJK/emoji/zero-width handling.
 pub fn wrap_line_chars(line: &str, width: usize) -> Vec<String> {
+    use unicode_width::UnicodeWidthChar;
+
     if width == 0 {
         return vec![String::new()];
     }
@@ -15,7 +18,15 @@ pub fn wrap_line_chars(line: &str, width: usize) -> Vec<String> {
     let mut row = String::new();
     let mut col = 0;
     for ch in line.chars() {
-        let w = if ch == '\t' { 4 } else { 1 };
+        let w = if ch == '\t' {
+            4
+        } else if ch.is_control() {
+            // Control characters (except tab): width 0, still pushed to preserve content.
+            // Visible rendering is the caller's responsibility.
+            0
+        } else {
+            ch.width().unwrap_or(0)
+        };
         if col + w > width && !row.is_empty() {
             result.push(std::mem::take(&mut row));
             col = 0;
@@ -132,4 +143,48 @@ mod tests {
     fn test_max_wrap_input_len_constant() {
         assert_eq!(MAX_WRAP_INPUT_LEN, 10 * 1024 * 1024);
     }
+
+    #[test]
+    fn test_wrap_cjk_chars() {
+        let result = wrap_line_chars("你好", 3);
+        assert_eq!(result, vec!["你", "好"]);
+    }
+
+    #[test]
+    fn test_wrap_cjk_ascii_mixed() {
+        let result = wrap_line_chars("a你好", 4);
+        assert_eq!(result, vec!["a你", "好"]);
+    }
+
+    #[test]
+    fn test_wrap_zero_width_char() {
+        let result = wrap_line_chars("a\u{200B}b", 2);
+        assert_eq!(result, vec!["a\u{200B}b"]);
+    }
+
+    #[test]
+    fn test_wrap_emoji() {
+        let result = wrap_line_chars("😀a", 3);
+        assert_eq!(result, vec!["😀a"]);
+    }
+
+    #[test]
+    fn test_wrap_emoji_exact_wrap() {
+        let result = wrap_line_chars("😀a", 2);
+        assert_eq!(result, vec!["😀", "a"]);
+    }
+
+    #[test]
+    fn test_wrap_combining_mark() {
+        // Scalar-width wrapping: combining mark (width 0) stays with next base char,
+        // not the preceding one, because the base char already triggered a flush.
+        let result = wrap_line_chars("a\u{0301}b", 1);
+        assert_eq!(result, vec!["a", "\u{0301}b"]);
+    }
+
+    #[test]
+    fn test_wrap_cjk_width_one() {
+        let result = wrap_line_chars("你好", 1);
+        assert_eq!(result, vec!["你", "好"]);
+    }
 }

crates/core/src/parser/json.rs

@@ -34,6 +34,19 @@ use serde_json::Value;
 // types 模块中定义了 LogEntry（一条日志记录）和 LogLevel（日志级别，如 INFO/ERROR）。
 use crate::types::{LogEntry, LogLevel};
 
+// ─── strip_bom 辅助函数 ──────────────────────────────────────────────────
+// 剥离行首的 UTF-8 BOM（Byte Order Mark, U+FEFF）。
+//
+// Windows 环境和某些导出工具生成的文件会在行首插入 BOM，
+// 而 serde_json 不接受 BOM 前缀的 JSON 文本（会报 "expected value" 错误）。
+// 只剥离一个前导 BOM，不处理多个 BOM 或行内 BOM（那些是畸形输入）。
+//
+// 参数: line: &str — 输入字符串切片。
+// 返回: &str — 去掉 BOM 后的字符串切片（借用原始字符串，零分配）。
+fn strip_bom(line: &str) -> &str {
+    line.strip_prefix('\u{FEFF}').unwrap_or(line)
+}
+
 // ─── detect_json_log 函数 ──────────────────────────────────────────────────
 // 检测一行文本是否是一个 JSON 对象。
 //
@@ -63,7 +76,38 @@ pub fn detect_json_log(line: &str) -> bool {
     //    则匹配成功。_ 是通配符，表示"不关心对象里面的具体内容"。
     //
     // 如果匹配到 Ok(Value::Object(_)) 返回 true，否则返回 false。
-    matches!(serde_json::from_str::<Value>(line), Ok(Value::Object(_)))
+    matches!(serde_json::from_str::<Value>(strip_bom(line)), Ok(Value::Object(_)))
+}
+
+// ─── take_string_field 辅助函数 ──────────────────────────────────────────────
+// 从 HashMap 中安全提取字符串字段。
+//
+// 遍历候选键名列表，找到第一个值为字符串类型的字段，移除并返回其值。
+// 如果值不是字符串（如数字、布尔值等），保留该字段在 HashMap 中，继续尝试下一个候选键。
+// 这样可以避免非字符串类型的字段被静默丢弃（数据丢失 bug）。
+//
+// 参数:
+//   - fields: &mut HashMap<String, Value> — JSON 字段的 HashMap（可变引用）。
+//   - keys: &[&str] — 候选键名列表（按优先级排列）。
+// 返回: Option<String> — 找到字符串值返回 Some(String)，否则返回 None。
+fn take_string_field(fields: &mut HashMap<String, Value>, keys: &[&str]) -> Option<String> {
+    for key in keys {
+        // 先检查值是否为字符串类型（peek，不移除）。
+        // is_some_and(Value::is_string) 等价于：
+        //   match fields.get(*key) {
+        //       Some(Value::String(_)) => true,
+        //       _ => false,
+        //   }
+        if fields.get(*key).is_some_and(Value::is_string) {
+            // 确认是字符串后才移除，取出 owned String（无需 clone）。
+            // unreachable! 在这里永远不会触发，因为我们刚刚确认了值的类型。
+            let Some(Value::String(value)) = fields.remove(*key) else {
+                unreachable!("value was checked as string");
+            };
+            return Some(value);
+        }
+    }
+    None
 }
 
 // ─── parse_line 函数 ──────────────────────────────────────────────────────
@@ -74,6 +118,8 @@ pub fn detect_json_log(line: &str) -> bool {
 //   Option 是 Rust 的可选类型：Some(值) 表示有值，None 表示没有值。
 pub fn parse_line(line: &str) -> Option<LogEntry> {
     // ─── 跳过空行 ──────────────────────────────────────────────────────────
+    let line = strip_bom(line);
+
     // line.trim() 去除首尾空白字符（空格、制表符、换行符等）。
     // .is_empty() 检查是否为空字符串。
     // 如果去除空白后是空的，说明是空行，不需要解析，直接返回 None。
@@ -104,57 +150,13 @@ pub fn parse_line(line: &str) -> Option<LogEntry> {
     let raw_line = line.to_string();
 
     // ─── 提取时间戳字段 ──────────────────────────────────────────────────
-    // 这段代码尝试从 JSON 中提取时间戳，逻辑如下：
-    //
-    // 1. ["timestamp", "time", "ts", "@timestamp"] — 候选键名数组。
-    //    不同日志系统使用不同的时间戳字段名，这里列出常见的几种。
-    //
-    // 2. .iter() — 创建数组的迭代器，可以逐个遍历元素。
-    //
-    // 3. .find_map(|key| fields.remove(*key)) — 对每个候选键名：
-    //    - fields.remove(*key): 尝试从 HashMap 中移除该键并返回对应的值。
-    //      如果键不存在，remove 返回 None。
-    //      *key 是解引用（deref），将 &str（引用）转换为 str，因为 remove 接受 &str 类型。
-    //    - find_map: 遍历所有候选键，返回第一个 Some(值) 的结果。
-    //      即找到第一个存在的键就停止。
-    //
-    // 4. .and_then(|v| v.as_str().map(String::from)) — 如果找到了时间戳值：
-    //    - v.as_str(): 尝试将 serde_json::Value 转换为 &str（字符串切片）。
-    //      如果 Value 不是字符串类型（比如是数字），返回 None。
-    //    - .map(String::from): 如果是字符串，将其转换为 String（拥有所有权的字符串）。
-    //    - and_then: 类似于 map，但用于"扁平化"嵌套的 Option。
-    //      如果 as_str() 返回 None，整个链返回 None。
-    let timestamp = ["timestamp", "time", "ts", "@timestamp"]
-        .iter()
-        .find_map(|key| fields.remove(*key))
-        .and_then(|v| v.as_str().map(String::from));
+    // 使用 take_string_field 辅助函数安全提取字符串类型的时间戳。
+    // 只有值为字符串时才会从 fields 中移除；非字符串值（如数字时间戳）保留在 fields 中。
+    let timestamp = take_string_field(&mut fields, &["timestamp", "time", "ts", "@timestamp"]);
 
     // ─── 提取日志级别字段 ──────────────────────────────────────────────
     // 与时间戳提取类似，但多了一步：将字符串解析为 LogLevel 枚举。
-    let level = ["level", "lvl", "severity"]
-        .iter()
-        .find_map(|key| fields.remove(*key))
-        .and_then(|v| v.as_str().map(String::from))
-        // .map(|s| s.parse::<LogLevel>(...)) — 尝试将字符串解析为 LogLevel 枚举。
-        // parse::<LogLevel> 中的 ::<LogLevel> 是泛型参数（turbofish 语法），
-        // 指定我们要将字符串解析为 LogLevel 类型。
-        //
-        // .unwrap_or_else(|e| match e {}) — 错误处理：
-        //   - 如果解析成功，直接返回 LogLevel 值。
-        //   - 如果解析失败（字符串不匹配任何已知的日志级别），执行闭包。
-        //   - |e| match e {}: 这个闭包接收解析错误 e，用 match e {} 进行"穷尽匹配"。
-        //     由于 LogLevel 的 parse 错误类型是一个空枚举（没有任何变体），
-        //     match e {} 意味着"这个分支永远不会执行"（unreachable）。
-        //     但实际上，如果 parse 失败，unwrap_or_else 不会执行这个闭包——
-        //     等等，这里有个细微之处：
-        //     unwrap_or_else 只在 Err 时执行闭包，但 match e {} 对空枚举是合法的
-        //     （因为空枚举没有任何可能的值，所以 match 是穷尽的）。
-        //     不过这里的实际效果是：如果 parse 失败，整个 .map() 返回 None
-        //     （因为 unwrap_or_else 返回的类型是 LogLevel，而空 match 不会有返回值）。
-        //
-        //     实际上更准确的解释：parse() 的错误类型是 Infallible（不可失败的），
-        //     即解析总是成功。所以 unwrap_or_else 永远不会被执行。
-        //     但即使如此，unwrap_or_else 的闭包也需要类型正确，match e {} 满足这一点。
+    let level = take_string_field(&mut fields, &["level", "lvl", "severity"])
         .map(|s| s.parse::<LogLevel>().unwrap_or_else(|e| match e {}));
 
     // ─── 构建 LogEntry 并返回 ──────────────────────────────────────────
@@ -357,4 +359,101 @@ mod tests {
             assert_eq!(entry.level, Some(LogLevel::Info), "failed for key: {key}");
         }
     }
+
+    #[test]
+    // 测试：数字类型的 level 值不应被提取，应保留在 fields 中。
+    fn test_numeric_level_preserved_in_fields() {
+        let line = r#"{"level":30,"message":"hello"}"#;
+        let entry = parse_line(line).unwrap();
+        // level 不是字符串，应返回 None。
+        assert!(entry.level.is_none());
+        // 数字 level 应保留在 fields 中，不被静默丢弃。
+        assert_eq!(entry.fields.get("level"), Some(&Value::Number(30.into())));
+        // message 仍正常存在。
+        assert_eq!(
+            entry.fields.get("message"),
+            Some(&Value::String("hello".to_string()))
+        );
+    }
+
+    #[test]
+    // 测试：数字类型的 timestamp 值不应被提取，应保留在 fields 中。
+    fn test_numeric_timestamp_preserved_in_fields() {
+        let line = r#"{"timestamp":1718000000,"message":"hello"}"#;
+        let entry = parse_line(line).unwrap();
+        // timestamp 不是字符串，应返回 None。
+        assert!(entry.timestamp.is_none());
+        // 数字 timestamp 应保留在 fields 中。
+        assert_eq!(
+            entry.fields.get("timestamp"),
+            Some(&Value::Number(1718000000.into()))
+        );
+    }
+
+    #[test]
+    // 测试：当第一个候选键是数字时，应回退到下一个字符串类型的候选键。
+    fn test_fallback_to_string_key() {
+        let line = r#"{"level":30,"lvl":"INFO","message":"hello"}"#;
+        let entry = parse_line(line).unwrap();
+        // "level" 是数字，应跳过；"lvl" 是字符串，应成功提取。
+        assert_eq!(entry.level, Some(LogLevel::Info));
+        // 数字 "level" 保留在 fields 中。
+        assert_eq!(entry.fields.get("level"), Some(&Value::Number(30.into())));
+        // "lvl" 已被成功提取并从 fields 中移除。
+        assert!(entry.fields.get("lvl").is_none());
+    }
+
+    #[test]
+    // 测试：timestamp 的 fallback 行为 — 数字 timestamp 被保留，字符串 time 被提取。
+    fn test_timestamp_fallback_preserves_numeric() {
+        let line = r#"{"timestamp":1718000000,"time":"2024-01-01T00:00:00Z"}"#;
+        let entry = parse_line(line).unwrap();
+        // "timestamp" 是数字，跳过；"time" 是字符串，成功提取。
+        assert_eq!(entry.timestamp, Some("2024-01-01T00:00:00Z".to_string()));
+        // 数字 "timestamp" 保留在 fields 中。
+        assert_eq!(
+            entry.fields.get("timestamp"),
+            Some(&Value::Number(1718000000.into()))
+        );
+        // "time" 已被提取并从 fields 中移除。
+        assert!(entry.fields.get("time").is_none());
+    }
+
+    #[test]
+    fn test_bom_prefixed_json() {
+        let line = "\u{FEFF}{\"level\":\"INFO\",\"message\":\"hello\"}";
+        let entry = parse_line(line).unwrap();
+        assert_eq!(entry.level, Some(LogLevel::Info));
+        assert_eq!(
+            entry.fields.get("message"),
+            Some(&Value::String("hello".into()))
+        );
+    }
+
+    #[test]
+    fn test_bom_prefixed_detect() {
+        assert!(detect_json_log("\u{FEFF}{\"level\":\"INFO\"}"));
+    }
+
+    #[test]
+    fn test_bom_only_whitespace() {
+        assert!(parse_line("\u{FEFF}   ").is_none());
+    }
+
+    #[test]
+    fn test_bom_stripped_from_raw_line() {
+        let line = "\u{FEFF}{\"level\":\"INFO\",\"message\":\"hello\"}";
+        let entry = parse_line(line).unwrap();
+        assert_eq!(entry.raw_line, "{\"level\":\"INFO\",\"message\":\"hello\"}");
+    }
+
+    #[test]
+    fn test_internal_bom_not_stripped() {
+        let line = "{\"message\":\"\u{FEFF}hello\"}";
+        let entry = parse_line(line).unwrap();
+        assert_eq!(
+            entry.fields.get("message"),
+            Some(&Value::String("\u{FEFF}hello".into()))
+        );
+    }
 }

crates/core/src/watcher/file_watcher.rs

@@ -13,6 +13,7 @@ pub enum FileEvent {
     Truncated { new_size: u64 },
     Rotated { new_inode: u64 },
     Removed,
+    WatcherError { message: String },
 }
 
 // ─── get_inode ──────────────────────────────────────────────────────────────
@@ -34,6 +35,43 @@ struct WatchState {
     last_inode: u64,
 }
 
+fn process_event(event: Event, watch_path: &Path, state: &mut WatchState) -> Option<FileEvent> {
+    if !event.paths.iter().any(|p| p == watch_path) {
+        return None;
+    }
+
+    match event.kind {
+        EventKind::Modify(_) | EventKind::Create(_) | EventKind::Any => {}
+        EventKind::Remove(_) => return Some(FileEvent::Removed),
+        _ => return None,
+    }
+
+    let current_inode = get_inode(watch_path).unwrap_or(0);
+    let current_size = std::fs::metadata(watch_path).map(|m| m.len()).unwrap_or(0);
+
+    if current_inode != 0 && state.last_inode != 0 && current_inode != state.last_inode {
+        state.last_inode = current_inode;
+        state.last_size = current_size;
+        Some(FileEvent::Rotated {
+            new_inode: current_inode,
+        })
+    } else if current_size > state.last_size {
+        state.last_size = current_size;
+        state.last_inode = current_inode;
+        Some(FileEvent::Appended {
+            new_size: current_size,
+        })
+    } else if current_size < state.last_size {
+        state.last_size = current_size;
+        state.last_inode = current_inode;
+        Some(FileEvent::Truncated {
+            new_size: current_size,
+        })
+    } else {
+        None
+    }
+}
+
 // ─── FileWatcher ────────────────────────────────────────────────────────────
 pub struct FileWatcher {
     rx: Receiver<FileEvent>,
@@ -56,24 +94,13 @@ impl FileWatcher {
             notify::recommended_watcher(move |res: std::result::Result<Event, notify::Error>| {
                 let event = match res {
                     Ok(e) => e,
-                    Err(_) => return,
-                };
-
-                match event.kind {
-                    EventKind::Modify(_) | EventKind::Create(_) | EventKind::Any => {}
-                    EventKind::Remove(_) => {
-                        let _ = tx.try_send(FileEvent::Removed);
+                    Err(error) => {
+                        let _ = tx.try_send(FileEvent::WatcherError {
+                            message: error.to_string(),
+                        });
                         return;
                     }
-                    _ => return,
-                }
-
-                if !event.paths.iter().any(|p| p == &watch_path) {
-                    return;
-                }
-
-                let current_inode = get_inode(&watch_path).unwrap_or(0);
-                let current_size = std::fs::metadata(&watch_path).map(|m| m.len()).unwrap_or(0);
+                };
 
                 let mut st = state.lock().unwrap_or_else(|poison| {
                     // Recover from poisoned mutex — state only tracks last_size
@@ -81,25 +108,8 @@ impl FileWatcher {
                     // cause a duplicate event, which is harmless.
                     poison.into_inner()
                 });
-
-                if current_inode != 0 && st.last_inode != 0 && current_inode != st.last_inode {
-                    let _ = tx.try_send(FileEvent::Rotated {
-                        new_inode: current_inode,
-                    });
-                    st.last_inode = current_inode;
-                    st.last_size = current_size;
-                } else if current_size > st.last_size {
-                    let _ = tx.try_send(FileEvent::Appended {
-                        new_size: current_size,
-                    });
-                    st.last_size = current_size;
-                    st.last_inode = current_inode;
-                } else if current_size < st.last_size {
-                    let _ = tx.try_send(FileEvent::Truncated {
-                        new_size: current_size,
-                    });
-                    st.last_size = current_size;
-                    st.last_inode = current_inode;
+                if let Some(fe) = process_event(event, &watch_path, &mut st) {
+                    let _ = tx.try_send(fe);
                 }
             })?;
 
@@ -144,6 +154,56 @@ mod tests {
         events
     }
 
+    #[test]
+    fn test_remove_wrong_path_ignored() {
+        let dir = tempfile::tempdir().expect("create temp dir");
+        let watched = dir.path().join("watched.log");
+        let other = dir.path().join("other.log");
+        std::fs::write(&watched, b"hello\n").expect("write watched");
+        std::fs::write(&other, b"other\n").expect("write other");
+
+        let mut state = WatchState {
+            last_size: 6,
+            last_inode: get_inode(&watched).unwrap_or(0),
+        };
+
+        let event = Event {
+            kind: EventKind::Remove(notify::event::RemoveKind::File),
+            paths: vec![other.clone()],
+            attrs: Default::default(),
+        };
+
+        let result = process_event(event, &watched, &mut state);
+        assert_eq!(
+            result, None,
+            "Remove for non-watched path should be ignored"
+        );
+    }
+
+    #[test]
+    fn test_remove_correct_path_emits_removed() {
+        let dir = tempfile::tempdir().expect("create temp dir");
+        let watched = dir.path().join("watched.log");
+        std::fs::write(&watched, b"hello\n").expect("write watched");
+
+        let mut state = WatchState {
+            last_size: 6,
+            last_inode: get_inode(&watched).unwrap_or(0),
+        };
+
+        let event = Event {
+            kind: EventKind::Remove(notify::event::RemoveKind::File),
+            paths: vec![watched.clone()],
+            attrs: Default::default(),
+        };
+
+        let result = process_event(event, &watched, &mut state);
+        assert!(
+            matches!(result, Some(FileEvent::Removed)),
+            "Remove for watched path should emit Removed"
+        );
+    }
+
     #[test]
     fn test_watcher_append() {
         let dir = tempfile::tempdir().expect("create temp dir");
@@ -232,5 +292,19 @@ mod tests {
 
         let d = FileEvent::Rotated { new_inode: 42 };
         assert_ne!(a, d);
+
+        let e1 = FileEvent::WatcherError {
+            message: "io error".into(),
+        };
+        let e2 = FileEvent::WatcherError {
+            message: "io error".into(),
+        };
+        assert_eq!(e1, e2);
+        assert_ne!(
+            e1,
+            FileEvent::WatcherError {
+                message: "other".into()
+            }
+        );
     }
-}
+}

crates/tui/src/app.rs

@@ -857,6 +857,7 @@ impl App {
                 FileEvent::Removed => {
                     self.loading_state = AppLoadingState::Error("File has been deleted".into());
                 }
+                FileEvent::WatcherError { message: _ } => {}
             }
         }
     }
@@ -865,6 +866,7 @@ impl App {
         let width = self.get_content_width();
         match &mut self.loading_state {
             AppLoadingState::Ready { reader } => {
+                let old_reader_line_count = reader.line_count();
                 let status = reader.update_for_append();
                 match status {
                     Ok(
@@ -883,13 +885,24 @@ impl App {
                         };
                         let new_line_count = reader.line_count();
 
-                        if can_extend && new_line_count > old_line_count {
+                        if can_extend && old_line_count == old_reader_line_count {
                             if let log_viewer_core::io::progressive_reader::ReaderState::Ready {
                                 visual_height_index: Some(index),
                                 reader: fr,
                             } = &mut reader.state
                             {
-                                let mut new_heights = Vec::with_capacity(new_line_count - old_line_count);
+                                if old_line_count > 0 {
+                                    let last_old_line_text =
+                                        fr.get_line(old_line_count - 1).unwrap_or("");
+                                    let new_h = compute_line_visual_height(
+                                        last_old_line_text,
+                                        width,
+                                        self.json_format,
+                                    );
+                                    index.replace_last_line_height(new_h);
+                                }
+                                let mut new_heights =
+                                    Vec::with_capacity(new_line_count.saturating_sub(old_line_count));
                                 for i in old_line_count..new_line_count {
                                     let line_text = fr.get_line(i).unwrap_or("");
                                     new_heights.push(compute_line_visual_height(
@@ -2473,7 +2486,9 @@ plain text line
     }
 
     fn install_vhi(app: &mut App, heights: &[usize]) {
-        let vhi = VisualHeightIndex::build(heights);
+        let width = app.get_content_width();
+        let json_format = app.json_format;
+        let vhi = VisualHeightIndex::build(heights).with_params(json_format, width);
         if let AppLoadingState::Ready { reader } = &mut app.loading_state {
             if let log_viewer_core::io::progressive_reader::ReaderState::Ready {
                 visual_height_index,
@@ -2800,4 +2815,88 @@ plain text line
         cleanup(&path);
         assert!(result.is_ok());
     }
+
+    #[test]
+    fn test_append_no_trailing_newline_updates_last_line_height() {
+        let path = make_temp_file("abc");
+        let result = std::panic::catch_unwind(|| {
+            let mut app = App::new();
+            load_file_ready(&mut app, &path);
+            assert_eq!(app.total_lines(), 1);
+
+            app.content_width = 5;
+            install_vhi(&mut app, &[1usize]);
+
+            {
+                let vhi = app.get_visual_height_index().unwrap();
+                assert_eq!(vhi.visual_height_of_line(0), 1);
+                assert_eq!(vhi.total_visual_rows(), 1);
+            }
+
+            // Append content that extends line 0 (no trailing newline before)
+            // "abc" + "defgh\n" = "abcdefgh\n" → 8 chars in width 5 → wraps to 2 visual rows
+            // old_total=1, old_had_trailing=false → starts_new_line=false
+            // new_has_trailing=true, new_newlines=1 → added = 1-1 = 0
+            // Still 1 logical line, but line 0 text changed from "abc" to "abcdefgh"
+            {
+                use std::io::Write;
+                let mut f = std::fs::OpenOptions::new()
+                    .append(true)
+                    .open(&path)
+                    .unwrap();
+                f.write_all(b"defgh\n").unwrap();
+            }
+
+            std::thread::sleep(std::time::Duration::from_millis(500));
+            app.poll_file_watcher();
+
+            assert_eq!(app.total_lines(), 1,
+                "\"abcdefgh\\n\" has trailing newline → 1 logical line");
+
+            let vhi = app.get_visual_height_index().expect("VHI should still exist after append");
+            assert_eq!(vhi.visual_height_of_line(0), 2,
+                "line 0 height should be updated from 1 to 2 after extending 'abcdefgh' in width 5");
+            assert_eq!(vhi.total_visual_rows(), 2);
+            assert_eq!(vhi.cursor_to_first_visual_row(0), 0);
+            cleanup(&path);
+        });
+        assert!(result.is_ok());
+    }
+
+    #[test]
+    fn test_append_no_trailing_newline_no_new_lines_only_height_change() {
+        let path = make_temp_file("abc");
+        let result = std::panic::catch_unwind(|| {
+            let mut app = App::new();
+            load_file_ready(&mut app, &path);
+            assert_eq!(app.total_lines(), 1);
+
+            app.content_width = 5;
+            install_vhi(&mut app, &[1usize]);
+
+            // Append without adding any new line — just extends line 0
+            // "abc" + "def" = "abcdef" → 6 chars in width 5 → wraps to 2 visual rows, still 1 logical line
+            {
+                use std::io::Write;
+                let mut f = std::fs::OpenOptions::new()
+                    .append(true)
+                    .open(&path)
+                    .unwrap();
+                f.write_all(b"def").unwrap();
+            }
+
+            std::thread::sleep(std::time::Duration::from_millis(500));
+            app.poll_file_watcher();
+
+            assert_eq!(app.total_lines(), 1,
+                "no new logical line should be added");
+
+            let vhi = app.get_visual_height_index().expect("VHI should still exist");
+            assert_eq!(vhi.visual_height_of_line(0), 2,
+                "line 0 height should update even when no new lines added");
+            assert_eq!(vhi.total_visual_rows(), 2);
+            cleanup(&path);
+        });
+        assert!(result.is_ok());
+    }
 }