diff --git a/pyproject.toml b/pyproject.toml
index 3cbde15..72e618c 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -1,6 +1,6 @@
 [tool.poetry]
 name = "textual-webterm"
-version = "0.3.8"
+version = "0.3.9"
 description = "Serve terminal sessions over the web"
 authors = ["Will McGugan <will@textualize.io>"]
 license = "MIT"
diff --git a/src/textual_webterm/svg_exporter.py b/src/textual_webterm/svg_exporter.py
index 9f88c2a..d467284 100644
--- a/src/textual_webterm/svg_exporter.py
+++ b/src/textual_webterm/svg_exporter.py
@@ -224,7 +224,8 @@ def render_terminal_svg(
 
             # For horizontal box-drawing spans, use textLength to ensure correct width
             # This prevents gaps caused by font rendering of ─ being narrower than char_width
-            if _is_all_horizontal_box_drawing(text) and len(text) > 1:
+            # Use "mostly" check to handle occasional corrupted chars (like U+FFFD)
+            if _is_mostly_horizontal_box_drawing(text) and len(text) > 1:
                 span_width = columns * char_width
                 attrs.append(f'textLength="{span_width:.1f}"')
                 attrs.append('lengthAdjust="spacing"')
@@ -291,11 +292,17 @@ _HORIZONTAL_BOX_CHARS = {
 }
 
 
-def _is_all_horizontal_box_drawing(text: str) -> bool:
-    """Check if text consists entirely of horizontal box-drawing characters."""
+def _is_mostly_horizontal_box_drawing(text: str, threshold: float = 0.8) -> bool:
+    """Check if text is mostly horizontal box-drawing characters.
+
+    Returns True if at least threshold (default 80%) of chars are horizontal
+    box-drawing chars. This handles cases where terminal data has occasional
+    corrupted chars (like replacement char U+FFFD) mixed in.
+    """
     if not text:
         return False
-    return all(ord(c) in _HORIZONTAL_BOX_CHARS for c in text)
+    horizontal_count = sum(1 for c in text if ord(c) in _HORIZONTAL_BOX_CHARS)
+    return horizontal_count / len(text) >= threshold
 
 
 def _should_break_span(current_text: str, new_char: str) -> bool:
diff --git a/tests/test_svg_exporter.py b/tests/test_svg_exporter.py
index 6e495ef..96e472c 100644
--- a/tests/test_svg_exporter.py
+++ b/tests/test_svg_exporter.py
@@ -10,8 +10,8 @@ from textual_webterm.svg_exporter import (
     _build_row_spans,
     _color_to_hex,
     _escape_xml,
-    _is_all_horizontal_box_drawing,
     _is_box_drawing_vertical_or_corner,
+    _is_mostly_horizontal_box_drawing,
     _should_break_span,
     render_terminal_svg,
 )
@@ -389,26 +389,33 @@ class TestBoxDrawingHelpers:
         assert _should_break_span("─", "─") is False
         assert _should_break_span("━", "━") is False
 
-    def test_is_all_horizontal_box_drawing_empty(self) -> None:
+    def test_is_mostly_horizontal_box_drawing_empty(self) -> None:
         """Empty string returns False."""
-        assert _is_all_horizontal_box_drawing("") is False
+        assert _is_mostly_horizontal_box_drawing("") is False
 
-    def test_is_all_horizontal_box_drawing_normal_text(self) -> None:
+    def test_is_mostly_horizontal_box_drawing_normal_text(self) -> None:
         """Normal text returns False."""
-        assert _is_all_horizontal_box_drawing("Hello") is False
-        assert _is_all_horizontal_box_drawing("ABC") is False
+        assert _is_mostly_horizontal_box_drawing("Hello") is False
+        assert _is_mostly_horizontal_box_drawing("ABC") is False
 
-    def test_is_all_horizontal_box_drawing_horizontal_lines(self) -> None:
+    def test_is_mostly_horizontal_box_drawing_horizontal_lines(self) -> None:
         """Horizontal box chars return True."""
-        assert _is_all_horizontal_box_drawing("─") is True
-        assert _is_all_horizontal_box_drawing("───") is True
-        assert _is_all_horizontal_box_drawing("━━━") is True
-        assert _is_all_horizontal_box_drawing("═══") is True
+        assert _is_mostly_horizontal_box_drawing("─") is True
+        assert _is_mostly_horizontal_box_drawing("───") is True
+        assert _is_mostly_horizontal_box_drawing("━━━") is True
+        assert _is_mostly_horizontal_box_drawing("═══") is True
 
-    def test_is_all_horizontal_box_drawing_mixed(self) -> None:
-        """Mixed content returns False."""
-        assert _is_all_horizontal_box_drawing("─A─") is False
-        assert _is_all_horizontal_box_drawing("│──") is False  # vertical at start
+    def test_is_mostly_horizontal_box_drawing_with_corruption(self) -> None:
+        """Mostly horizontal with some corrupted chars returns True."""
+        # 90% horizontal (9 out of 10)
+        assert _is_mostly_horizontal_box_drawing("─────────X") is True
+        # With replacement chars (like U+FFFD)
+        assert _is_mostly_horizontal_box_drawing("───\ufffd───") is True
+
+    def test_is_mostly_horizontal_box_drawing_mixed(self) -> None:
+        """Mixed content below threshold returns False."""
+        assert _is_mostly_horizontal_box_drawing("─A─") is False  # 66% horizontal
+        assert _is_mostly_horizontal_box_drawing("│──") is False  # vertical at start
 
 
 class TestRenderTerminalSvg: