documents: GitHub alerts, a [toc] list of headings, front matter in a box, HTML blocks as text, a visible rule and bullets by depth

At the root a quote marked [!NOTE] and the rest, as GitHub's API reads it,
is an alert; a [toc] paragraph lists the headings and a press reveals one;
an HTML block shows its text with the inline tags, white space as a browser
lays it, scripts and styles dropped. The rule is 2px; bullets turn by depth.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

src/documents/document/tests/mod.rs

@@ -370,6 +370,7 @@ mod history;
 mod links;
 mod look;
 mod place;
+mod toc;
 mod tools;
 
 #[gpui::test]

src/documents/document/tests/toc.rs

@@ -0,0 +1,36 @@
+use gpui::{Modifiers, MouseButton, TestAppContext, VisualTestContext, point, px};
+
+use super::{MarkdownMode, open};
+
+const LISTED: &str = "[toc]\n\n# A\n\ntext\n\n## B\n";
+
+/// Presses entry `ix` of the drawn heading list.
+fn press_entry(ix: usize, modifiers: Modifiers, cx: &mut VisualTestContext) {
+    let bounds = cx
+        .debug_bounds(Box::leak(format!("markdown-toc-{ix}").into_boxed_str()))
+        .expect("the entry is drawn");
+    let at = point(bounds.left() + px(2.0), bounds.top() + px(2.0));
+    cx.simulate_mouse_down(at, MouseButton::Left, modifiers);
+    cx.simulate_mouse_up(at, MouseButton::Left, modifiers);
+    cx.run_until_parked();
+}
+
+#[gpui::test]
+fn a_press_in_preview_reveals_the_heading(cx: &mut TestAppContext) {
+    let (document, cx) = open(LISTED, MarkdownMode::Preview, cx);
+    press_entry(1, Modifiers::none(), cx);
+    let caret = document.read_with(cx, |document, cx| document.caret(cx));
+    assert_eq!(caret, (6, 0));
+}
+
+#[gpui::test]
+fn visual_reveals_on_a_secondary_press_and_edits_on_a_plain_one(cx: &mut TestAppContext) {
+    let (document, cx) = open(LISTED, MarkdownMode::Visual, cx);
+    press_entry(1, Modifiers::secondary_key(), cx);
+    let (caret, open) = document.read_with(cx, |document, cx| {
+        (document.caret(cx), document.open.is_some())
+    });
+    assert_eq!((caret, open), ((6, 0), false));
+    press_entry(0, Modifiers::none(), cx);
+    assert!(document.read_with(cx, |document, _| document.open.is_some()));
+}