markdown_preview: Improve nested list item prefix style (#39606)

Release Notes:

- Improved nested list item prefix style for Markdown preview.


## Before

<img width="667" height="450" alt="SCR-20251006-rtis"
src="https://github.com/user-attachments/assets/439160c4-7982-463c-9017-268d47c42c0c"
/>

## After

<img width="739" height="440" alt="SCR-20251006-rzlb"
src="https://github.com/user-attachments/assets/f6c237d9-3ff0-4468-ae9c-6853c5c2946a"
/>

---------

Co-authored-by: Bennet Bo Fenner <bennetbo@gmx.de>

crates/markdown_preview/src/markdown_renderer.rs

@@ -233,10 +233,11 @@ fn render_markdown_list_item(
     cx: &mut RenderContext,
 ) -> AnyElement {
     use ParsedMarkdownListItemType::*;
+    let depth = parsed.depth.saturating_sub(1) as usize;
 
     let bullet = match &parsed.item_type {
-        Ordered(order) => format!("{}.", order).into_any_element(),
-        Unordered => "•".into_any_element(),
+        Ordered(order) => list_item_prefix(*order as usize, true, depth).into_any_element(),
+        Unordered => list_item_prefix(1, false, depth).into_any_element(),
         Task(checked, range) => div()
             .id(cx.next_id(range))
             .mt(cx.scaled_rems(3.0 / 16.0))
@@ -292,10 +293,8 @@ fn render_markdown_list_item(
         .collect();
 
     let item = h_flex()
-        .when(!parsed.nested, |this| {
-            this.pl(cx.scaled_rems(parsed.depth.saturating_sub(1) as f32))
-        })
-        .when(parsed.nested && parsed.depth > 1, |this| this.ml_neg_1p5())
+        .when(!parsed.nested, |this| this.pl(cx.scaled_rems(depth as f32)))
+        .when(parsed.nested && depth > 0, |this| this.ml_neg_1p5())
         .items_start()
         .children(vec![
             bullet,
@@ -880,6 +879,38 @@ impl Render for InteractiveMarkdownElementTooltip {
     }
 }
 
+/// Returns the prefix for a list item.
+fn list_item_prefix(order: usize, ordered: bool, depth: usize) -> String {
+    let ix = order.saturating_sub(1);
+    const NUMBERED_PREFIXES_1: &str = "ABCDEFGHIJKLMNOPQRSTUVWXYZ";
+    const NUMBERED_PREFIXES_2: &str = "abcdefghijklmnopqrstuvwxyz";
+    const BULLETS: [&str; 5] = ["•", "◦", "▪", "‣", "⁃"];
+
+    if ordered {
+        match depth {
+            0 => format!("{}. ", order),
+            1 => format!(
+                "{}. ",
+                NUMBERED_PREFIXES_1
+                    .chars()
+                    .nth(ix % NUMBERED_PREFIXES_1.len())
+                    .unwrap()
+            ),
+            _ => format!(
+                "{}. ",
+                NUMBERED_PREFIXES_2
+                    .chars()
+                    .nth(ix % NUMBERED_PREFIXES_2.len())
+                    .unwrap()
+            ),
+        }
+    } else {
+        let depth = depth.min(BULLETS.len() - 1);
+        let bullet = BULLETS[depth];
+        return format!("{} ", bullet);
+    }
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;
@@ -1020,4 +1051,25 @@ mod tests {
             ])
         );
     }
+
+    #[test]
+    fn test_list_item_prefix() {
+        assert_eq!(list_item_prefix(1, true, 0), "1. ");
+        assert_eq!(list_item_prefix(2, true, 0), "2. ");
+        assert_eq!(list_item_prefix(3, true, 0), "3. ");
+        assert_eq!(list_item_prefix(11, true, 0), "11. ");
+        assert_eq!(list_item_prefix(1, true, 1), "A. ");
+        assert_eq!(list_item_prefix(2, true, 1), "B. ");
+        assert_eq!(list_item_prefix(3, true, 1), "C. ");
+        assert_eq!(list_item_prefix(1, true, 2), "a. ");
+        assert_eq!(list_item_prefix(2, true, 2), "b. ");
+        assert_eq!(list_item_prefix(7, true, 2), "g. ");
+        assert_eq!(list_item_prefix(1, true, 1), "A. ");
+        assert_eq!(list_item_prefix(1, true, 2), "a. ");
+        assert_eq!(list_item_prefix(1, false, 0), "• ");
+        assert_eq!(list_item_prefix(1, false, 1), "◦ ");
+        assert_eq!(list_item_prefix(1, false, 2), "▪ ");
+        assert_eq!(list_item_prefix(1, false, 3), "‣ ");
+        assert_eq!(list_item_prefix(1, false, 4), "⁃ ");
+    }
 }