perf: speed up large markdown rendering

crates/editor/tests/markdown.rs

@@ -447,3 +447,50 @@ fn parse_body(source: &str) -> Vec<MarkdownBlock> {
     let state = MarkdownViewState::new("parse-md", source);
     state.blocks().to_vec()
 }
+/// Mirrors the gallery's "Large file" sample: five blocks per section.
+fn large_markdown(sections: usize) -> String {
+    let mut source = String::new();
+    for section in 0..sections {
+        source.push_str(&format!("## Section {section}\n\n"));
+        source.push_str(&format!(
+            "Handler {section} retries stale sockets and logs every outcome.\n\n"
+        ));
+        source.push_str(&format!(
+            "- [ ] Audit handler {section}\n- See [issue {section}](https://example.com/issues/{section})\n\n"
+        ));
+        source.push_str("```rust\nfn handle() -> bool { true }\n```\n\n");
+    }
+    source
+}
+
+#[test]
+fn a_large_document_swaps_and_renders_without_freezing() {
+    let source = large_markdown(1000);
+    let started = std::time::Instant::now();
+
+    let (_cx, mut visual, state) = view_window("# small\n");
+    state.update_in(&mut visual, |state, _window, cx| {
+        state.set_source(source, cx)
+    });
+    visual.update(|window, cx| {
+        window.draw(cx).clear(cx);
+    });
+
+    let elapsed = started.elapsed();
+    assert!(
+        elapsed < std::time::Duration::from_secs(15),
+        "swapping to a large document took {elapsed:?}; the frame must not
+         measure every block"
+    );
+
+    let count = _cx.update(|cx| state.read(cx).blocks().len());
+    assert_eq!(count, 5000, "five blocks per section");
+    assert!(
+        visual.debug_bounds("test-md-block-0").is_some(),
+        "the first block renders after the swap"
+    );
+    assert!(
+        visual.debug_bounds("test-md-block-4999").is_none(),
+        "blocks past the viewport never render"
+    );
+}