diff --git a/crates/fresh-editor/src/model/buffer/mod.rs b/crates/fresh-editor/src/model/buffer/mod.rs
index 82e6de63be..2f76912a48 100644
--- a/crates/fresh-editor/src/model/buffer/mod.rs
+++ b/crates/fresh-editor/src/model/buffer/mod.rs
@@ -1717,6 +1717,13 @@ impl TextBuffer {
             line_feed_cnt,
             &self.buffers,
         );
+
+        // Content changed: layout caches (line-wrap cache, visual-row
+        // index) key on the version and would otherwise serve
+        // pre-append state.  Deliberately not `mark_content_modified`
+        // — the buffer still matches the on-disk stream, so it isn't
+        // dirty.
+        self.bump_version();
     }
 
     /// Check if the buffer has been modified since last save
diff --git a/crates/fresh-editor/src/primitives/grapheme.rs b/crates/fresh-editor/src/primitives/grapheme.rs
index d90a31b211..bdb72f8953 100644
--- a/crates/fresh-editor/src/primitives/grapheme.rs
+++ b/crates/fresh-editor/src/primitives/grapheme.rs
@@ -9,7 +9,20 @@
 //! A grapheme cluster is what a user perceives as a single character.
 //! For example, Thai "ที่" looks like one character but is 3 Unicode code points.
 
-use unicode_segmentation::UnicodeSegmentation;
+use unicode_segmentation::{GraphemeCursor, UnicodeSegmentation};
+
+/// Snap `pos` down to a UTF-8 code-point boundary so it can be handed to
+/// [`GraphemeCursor`], which requires boundary-aligned offsets. A position
+/// inside a code point can never be a grapheme boundary, so snapping down
+/// preserves the semantics of every caller below.
+#[inline]
+fn snap_to_char_boundary(s: &str, mut pos: usize) -> usize {
+    pos = pos.min(s.len());
+    while pos > 0 && !s.is_char_boundary(pos) {
+        pos -= 1;
+    }
+    pos
+}
 
 /// Find the byte position of the previous grapheme cluster boundary.
 ///
@@ -28,18 +41,19 @@ pub fn prev_grapheme_boundary(s: &str, pos: usize) -> usize {
         return 0;
     }
 
-    let pos = pos.min(s.len());
-
-    // Find all grapheme boundaries up to our position
-    let mut last_boundary = 0;
-    for (idx, _) in s.grapheme_indices(true) {
-        if idx >= pos {
-            break;
-        }
-        last_boundary = idx;
+    // A position INSIDE a code point is inside that code point's
+    // cluster: the boundary strictly before it is the containing
+    // cluster's START, which the snapped char boundary may already be
+    // (single-code-point cluster).  Stepping `prev_boundary` from the
+    // snapped position instead would overshoot one cluster left.
+    let snapped = snap_to_char_boundary(s, pos);
+    if snapped < pos {
+        return snap_to_grapheme_boundary(s, snapped);
     }
-
-    last_boundary
+    let mut cursor = GraphemeCursor::new(snapped, s.len(), true);
+    // With the whole string as a single chunk (chunk_start == 0) the cursor
+    // never needs pre-context, so this cannot fail.
+    cursor.prev_boundary(s, 0).ok().flatten().unwrap_or(0)
 }
 
 /// Find the byte position of the next grapheme cluster boundary.
@@ -59,20 +73,11 @@ pub fn next_grapheme_boundary(s: &str, pos: usize) -> usize {
         return s.len();
     }
 
-    // Find the grapheme that contains our position, then return its end
-    for (idx, grapheme) in s.grapheme_indices(true) {
-        let end = idx + grapheme.len();
-        if idx >= pos {
-            // This grapheme starts at or after our position
-            return end;
-        }
-        if end > pos {
-            // Our position is within this grapheme
-            return end;
-        }
-    }
-
-    s.len()
+    // A mid-cluster position (snapped or not) advances to the end of the
+    // containing cluster — the next boundary after it.
+    let pos = snap_to_char_boundary(s, pos);
+    let mut cursor = GraphemeCursor::new(pos, s.len(), true);
+    cursor.next_boundary(s, 0).ok().flatten().unwrap_or(s.len())
 }
 
 /// Get the grapheme cluster at the given position.
@@ -87,14 +92,9 @@ pub fn grapheme_at(s: &str, pos: usize) -> Option<(&str, usize, usize)> {
         return None;
     }
 
-    for (idx, grapheme) in s.grapheme_indices(true) {
-        let end = idx + grapheme.len();
-        if idx <= pos && pos < end {
-            return Some((grapheme, idx, end));
-        }
-    }
-
-    None
+    let start = snap_to_grapheme_boundary(s, pos);
+    let end = next_grapheme_boundary(s, start);
+    Some((&s[start..end], start, end))
 }
 
 /// Snap a byte position **down** to the nearest grapheme-cluster
@@ -120,17 +120,13 @@ pub fn snap_to_grapheme_boundary(s: &str, pos: usize) -> usize {
     if pos >= s.len() {
         return s.len();
     }
-    let mut last_boundary = 0;
-    for (idx, _) in s.grapheme_indices(true) {
-        if idx == pos {
-            return pos;
-        }
-        if idx > pos {
-            break;
-        }
-        last_boundary = idx;
+    let pos = snap_to_char_boundary(s, pos);
+    let mut cursor = GraphemeCursor::new(pos, s.len(), true);
+    match cursor.is_boundary(s, 0) {
+        Ok(true) => pos,
+        // Not a boundary → start of the containing cluster.
+        _ => cursor.prev_boundary(s, 0).ok().flatten().unwrap_or(0),
     }
-    last_boundary
 }
 
 /// Count the number of grapheme clusters in a string.
diff --git a/crates/fresh-editor/src/view/folding.rs b/crates/fresh-editor/src/view/folding.rs
index 9d0fbe49a5..da38abe348 100644
--- a/crates/fresh-editor/src/view/folding.rs
+++ b/crates/fresh-editor/src/view/folding.rs
@@ -396,27 +396,27 @@ impl Default for FoldManager {
 /// ([`PatternIndentCalculator::count_leading_indent`]).
 pub mod indent_folding {
     use crate::model::buffer::Buffer;
-    use crate::primitives::indent_pattern::PatternIndentCalculator;
+
+    /// Chunk size for the line-boundary scans below.  Scanning via
+    /// per-byte `byte_at` calls costs a full `slice_bytes` round-trip
+    /// per byte, which is what made cursor movement inside a very long
+    /// line slow (these scans run per rendered frame); block reads make
+    /// the scans effectively memchr-speed.
+    const LINE_SCAN_CHUNK: usize = 4096;
 
     /// Find the byte offset of the start of the line containing `pos`.
     /// Scans backward for `\n` (or returns 0).
     pub fn find_line_start_byte(buffer: &Buffer, pos: usize) -> usize {
-        if pos == 0 {
-            return 0;
-        }
-        let mut p = pos.min(buffer.len()).saturating_sub(1);
-        loop {
-            match PatternIndentCalculator::byte_at(buffer, p) {
-                Some(b'\n') => return p + 1,
-                None => return 0,
-                _ => {
-                    if p == 0 {
-                        return 0;
-                    }
-                    p -= 1;
-                }
+        let mut end = pos.min(buffer.len());
+        while end > 0 {
+            let start = end.saturating_sub(LINE_SCAN_CHUNK);
+            let bytes = buffer.slice_bytes(start..end);
+            if let Some(i) = bytes.iter().rposition(|&b| b == b'\n') {
+                return start + i + 1;
             }
+            end = start;
         }
+        0
     }
 
     /// Find the exclusive byte offset just past the line containing `pos`
@@ -424,13 +424,14 @@ pub mod indent_folding {
     /// line has no trailing newline). Scans forward for `\n`.
     pub fn find_line_end_byte(buffer: &Buffer, pos: usize) -> usize {
         let buf_len = buffer.len();
-        let mut p = pos;
-        while p < buf_len {
-            match PatternIndentCalculator::byte_at(buffer, p) {
-                Some(b'\n') => return p + 1,
-                None => return buf_len,
-                _ => p += 1,
+        let mut start = pos.min(buf_len);
+        while start < buf_len {
+            let end = (start + LINE_SCAN_CHUNK).min(buf_len);
+            let bytes = buffer.slice_bytes(start..end);
+            if let Some(i) = bytes.iter().position(|&b| b == b'\n') {
+                return start + i + 1;
             }
+            start = end;
         }
         buf_len
     }
@@ -582,16 +583,7 @@ pub mod indent_folding {
     /// Scans forward for `\n` and returns the byte after it. If no `\n` is
     /// found, returns `buffer.len()`.
     pub fn find_next_line_start_byte(buffer: &Buffer, pos: usize) -> usize {
-        let mut p = pos;
-        let len = buffer.len();
-        while p < len {
-            match PatternIndentCalculator::byte_at(buffer, p) {
-                Some(b'\n') => return p + 1,
-                None => return len,
-                _ => p += 1,
-            }
-        }
-        len
+        find_line_end_byte(buffer, pos)
     }
 
     /// Byte-range of a fold that contains `target_byte`.
diff --git a/crates/fresh-editor/tests/streaming_and_grapheme_regression_tests.rs b/crates/fresh-editor/tests/streaming_and_grapheme_regression_tests.rs
new file mode 100644
index 0000000000..d2b0146816
--- /dev/null
+++ b/crates/fresh-editor/tests/streaming_and_grapheme_regression_tests.rs
@@ -0,0 +1,77 @@
+//! Regression tests for two fixes that came out of the wrapped-line
+//! navigation performance review:
+//!
+//! * `extend_streaming` mutates buffer content (appends bytes) but did
+//!   not bump `buffer.version()`, so version-keyed layout caches
+//!   (visual-row index, line-wrap cache) could remain reachable with
+//!   pre-append state.
+//!
+//! * `prev_grapheme_boundary` stepped one cluster too far when given a
+//!   position INSIDE a multi-byte code point whose cluster is a single
+//!   code point (accented latin, CJK): the snapped char boundary IS the
+//!   containing cluster's start, and stepping `prev_boundary` from it
+//!   overshoots.
+
+mod common;
+
+use common::harness::EditorTestHarness;
+
+/// `extend_streaming` appends content and must bump `buffer.version()`
+/// like every other mutation path — layout caches key on the version,
+/// so an unchanged version can serve pre-append layout.
+#[test]
+fn extend_streaming_bumps_buffer_version() {
+    use std::io::Write;
+
+    let dir = tempfile::TempDir::new().unwrap();
+    let path = dir.path().join("stream.txt");
+    std::fs::write(&path, "hello\n").unwrap();
+
+    let mut harness = EditorTestHarness::new(80, 24).unwrap();
+    harness.open_file(&path).unwrap();
+    harness.render().unwrap();
+
+    let before = harness.editor().active_state().buffer.version();
+
+    // The stream grows on disk; the editor is told to extend.
+    let mut f = std::fs::OpenOptions::new()
+        .append(true)
+        .open(&path)
+        .unwrap();
+    f.write_all(b"appended\n").unwrap();
+    f.flush().unwrap();
+    let new_size = std::fs::metadata(&path).unwrap().len() as usize;
+    harness
+        .editor_mut()
+        .active_state_mut()
+        .buffer
+        .extend_streaming(&path, new_size);
+
+    let after = harness.editor().active_state().buffer.version();
+    assert_ne!(
+        before,
+        after,
+        "extend_streaming appended {} bytes but buffer.version() did not change — \
+         version-keyed layout caches will serve pre-append content",
+        new_size - 6,
+    );
+}
+
+/// For a position INSIDE a multi-byte code point,
+/// `prev_grapheme_boundary` must return the start of the CONTAINING
+/// cluster, not the boundary one cluster earlier.
+#[test]
+fn prev_grapheme_boundary_mid_code_point_returns_containing_cluster_start() {
+    use fresh::primitives::grapheme::prev_grapheme_boundary;
+
+    // "aé": 'a' at 0, 'é' occupies bytes 1..3.  Position 2 is inside
+    // 'é'; its containing cluster starts at 1.
+    assert_eq!(prev_grapheme_boundary("aé", 2), 1);
+
+    // Sanity: a position on a char boundary INSIDE a multi-code-point
+    // cluster still resolves to the cluster start...
+    assert_eq!(prev_grapheme_boundary("ที่", 3), 0);
+    // ...and a position on a cluster boundary steps to the previous one.
+    assert_eq!(prev_grapheme_boundary("aé", 1), 0);
+    assert_eq!(prev_grapheme_boundary("aé", 3), 1);
+}