fix(md): Fit thematic breaks inside quotes and lists

crates/jp_md/src/format_tests.rs

@@ -1342,6 +1342,22 @@ fn test_thematic_break_line_style_ignores_terminal_width() {
     );
 }
 
+#[test]
+fn test_thematic_break_line_style_fits_inside_a_blockquote() {
+    // A blockquote's `> ` narrows the text column by two. A rule that ignored
+    // the prefix would overflow the line and wrap.
+    let mut formatter = Formatter::with_width(40);
+    formatter.hr_style = HrStyle::Line;
+
+    let actual = formatter
+        .format_terminal("> above\n>\n> ---\n>\n> below")
+        .unwrap();
+    assert!(
+        actual.contains(&"─".repeat(38)) && !actual.contains(&"─".repeat(39)),
+        "Expected a 38-char unicode line inside the quote.\nActual: {actual:?}"
+    );
+}
+
 #[test]
 fn test_table_is_fitted_to_the_terminal_width() {
     // A table is laid out, not wrapped, so it has to fit the terminal on its

crates/jp_md/src/render.rs

@@ -503,8 +503,11 @@ impl<'a, 'w> TerminalFormatter<'a, 'w> {
             }
             HrStyle::Line => {
                 // A rule spans the text column, not the terminal, so it lines up
-                // with the wrapped prose above and below it.
-                let line: String = "─".repeat(self.writer.width.max(1));
+                // with the wrapped prose above and below it. Inside a
+                // blockquote or a list item that column is narrowed by the
+                // prefix each line carries.
+                let width = self.writer.width.saturating_sub(self.writer.prefix_width());
+                let line: String = "─".repeat(width.max(1));
                 self.writer.output(&line, false)?;
             }
         }