Fix horizontal box-drawing alignment with textLength attribute

Horizontal line characters (─━═) render narrower than the intended
character width in most fonts, causing gaps when followed by other
characters. Now using textLength + lengthAdjust='spacing' to force
horizontal box-drawing spans to occupy their correct width.

- Added _is_all_horizontal_box_drawing() helper
- Added textLength attribute for horizontal line spans > 1 char
- Added comprehensive tests for new functionality
- svg_exporter.py now has 100% test coverage

Version bump to 0.3.8

pyproject.toml

@@ -1,6 +1,6 @@
 [tool.poetry]
 name = "textual-webterm"
-version = "0.3.7"
+version = "0.3.8"
 description = "Serve terminal sessions over the web"
 authors = ["Will McGugan <will@textualize.io>"]
 license = "MIT"

src/textual_webterm/svg_exporter.py

@@ -222,6 +222,13 @@ def render_terminal_svg(
             if classes:
                 attrs.append(f'class="{" ".join(classes)}"')
 
+            # 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:
+                span_width = columns * char_width
+                attrs.append(f'textLength="{span_width:.1f}"')
+                attrs.append('lengthAdjust="spacing"')
+
             parts.append(f'<tspan {" ".join(attrs)}>{_escape_xml(text)}</tspan>')
             x += columns * char_width
 
@@ -274,6 +281,23 @@ def _is_box_drawing_vertical_or_corner(char: str) -> bool:
     return code not in horizontal_chars
 
 
+# Set of horizontal box-drawing character codes for span detection
+_HORIZONTAL_BOX_CHARS = {
+    0x2500, 0x2501,  # ─ ━
+    0x2504, 0x2505,  # ┄ ┅ (horizontal dashed)
+    0x2508, 0x2509,  # ┈ ┉ (horizontal dashed)
+    0x254C, 0x254D,  # ╌ ╍ (horizontal dashed)
+    0x2550,          # ═
+}
+
+
+def _is_all_horizontal_box_drawing(text: str) -> bool:
+    """Check if text consists entirely of horizontal box-drawing characters."""
+    if not text:
+        return False
+    return all(ord(c) in _HORIZONTAL_BOX_CHARS for c in text)
+
+
 def _should_break_span(current_text: str, new_char: str) -> bool:
     """Check if we should break the span before adding new_char.
 

tests/test_svg_exporter.py

@@ -10,6 +10,7 @@ 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,
     _should_break_span,
     render_terminal_svg,
@@ -387,6 +388,30 @@ class TestBoxDrawingHelpers:
         """Horizontal lines can merge with each other."""
         assert _should_break_span("─", "─") is False
         assert _should_break_span("━", "━") is False
+
+    def test_is_all_horizontal_box_drawing_empty(self) -> None:
+        """Empty string returns False."""
+        assert _is_all_horizontal_box_drawing("") is False
+
+    def test_is_all_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
+
+    def test_is_all_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
+
+    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
+
+
+class TestRenderTerminalSvg:
     """Tests for render_terminal_svg function."""
 
     def _char(
@@ -797,6 +822,20 @@ class TestEdgeCases:
         assert "─" in svg
         assert "┐" in svg
 
+    def test_horizontal_lines_use_textlength(self) -> None:
+        """Horizontal line spans use textLength for correct width."""
+        buffer = [[
+            self._char("╭"),
+            self._char("─"),
+            self._char("─"),
+            self._char("─"),
+            self._char("╮"),
+        ]]
+        svg = render_terminal_svg(buffer, width=5, height=1)
+        # Horizontal lines should have textLength attribute
+        assert 'textLength="24.0"' in svg
+        assert 'lengthAdjust="spacing"' in svg
+
     def test_ansi_bright_colors(self) -> None:
         """All bright ANSI colors render."""
         colors = ["brightred", "brightgreen", "brightyellow",