widgets: _text_area: Support zero-indexed line numbers

src/textual/widgets/_text_area.py

@@ -164,6 +164,13 @@ class TextArea(ScrollView):
     | `text-area--matching-bracket` | Target matching brackets. |
     """
 
+    LINE_NUMBER_START = 1
+    """
+    Allow a subclass to change the number lines starts at.
+    Gutter size will **not** be adjusted for negative signs, 0-indexed lines
+    will work as expected.
+    """
+
     BINDINGS = [
         # Cursor movement
         Binding("up", "cursor_up", "cursor up", show=False),
@@ -1133,7 +1140,9 @@ def render_line(self, y: int) -> Strip:
                 gutter_style = theme.gutter_style
 
             gutter_width_no_margin = gutter_width - 2
-            gutter_content = str(line_index + 1) if section_offset == 0 else ""
+            gutter_content = (
+                str(line_index + self.LINE_NUMBER_START) if section_offset == 0 else ""
+            )
             gutter = Text(
                 f"{gutter_content:>{gutter_width_no_margin}}  ",
                 style=gutter_style or "",
@@ -1458,7 +1467,8 @@ def gutter_width(self) -> int:
         # The longest number in the gutter plus two extra characters: `│ `.
         gutter_margin = 2
         gutter_width = (
-            len(str(self.document.line_count)) + gutter_margin
+            len(str(self.document.line_count - 1 + self.LINE_NUMBER_START))
+            + gutter_margin
             if self.show_line_numbers
             else 0
         )