Add 0.5px overlap to background rects for sub-pixel gap elimination

Background rects now extend 0.5px in both width and height to create
a slight overlap, eliminating visible sub-pixel gaps when viewing
SVG screenshots at high zoom levels.

pyproject.toml

@@ -1,6 +1,6 @@
 [tool.poetry]
 name = "textual-webterm"
-version = "0.3.12"
+version = "0.3.13"
 description = "Serve terminal sessions over the web"
 authors = ["Will McGugan <will@textualize.io>"]
 license = "MIT"

src/textual_webterm/svg_exporter.py

@@ -207,11 +207,12 @@ def render_terminal_svg(
                 char_cols = 2  # Wide character
 
             # Background rect if not default
+            # Add 0.5px overlap in both directions to eliminate sub-pixel gaps at high zoom
             if bg != background:
-                bg_width = char_cols * char_width
+                bg_width = char_cols * char_width + 0.5
                 row_bg_rects.append(
                     f'<rect x="{x:.1f}" y="{rect_y:.1f}" '
-                    f'width="{bg_width:.1f}" height="{actual_line_height:.1f}" '
+                    f'width="{bg_width:.1f}" height="{actual_line_height + 0.5:.1f}" '
                     f'fill="{bg}"/>'
                 )
 

tests/test_svg_exporter.py

@@ -300,15 +300,15 @@ class TestRenderTerminalSvg:
             self._char("", bg="red"),  # Placeholder inherits bg
         ]]
         svg = render_terminal_svg(buffer, width=80, height=24, char_width=10.0)
-        # Background should span 2 columns (20px width)
+        # Background should span 2 columns (20px width + 0.5px overlap)
         assert f'fill="{ANSI_COLORS["red"]}"' in svg
-        # Verify rect width is for 2 columns
+        # Verify rect width is for 2 columns plus overlap
         import re
         rect_match = re.search(r'<rect[^>]*width="(\d+\.?\d*)"[^>]*fill="{}"/>'
                               .format(ANSI_COLORS["red"]), svg)
         assert rect_match is not None
         width = float(rect_match.group(1))
-        assert width == 20.0  # 2 columns * 10.0 char_width
+        assert width == 20.5  # 2 columns * 10.0 char_width + 0.5 overlap
 
     def test_background_same_as_terminal_bg_no_rect(self) -> None:
         """Background same as terminal background doesn't create extra rect."""