fix(mermaid): strip Mermaid frontmatter before rendering (#10676)

app/src/notebooks/editor/model.rs

@@ -36,7 +36,10 @@ use string_offset::CharOffset;
 use warp_core::features::FeatureFlag;
 use warp_core::semantic_selection::SemanticSelection;
 use warp_editor::{
-    content::{buffer::ShouldAutoscroll, selection_model::BufferSelectionModel},
+    content::{
+        buffer::ShouldAutoscroll, mermaid_diagram::strip_mermaid_frontmatter,
+        selection_model::BufferSelectionModel,
+    },
     model::BufferUpdateWrapper,
     render::model::{BlockItem, StyleUpdateAction},
 };
@@ -154,7 +157,8 @@ fn mermaid_image_html(svg: &[u8]) -> String {
 }
 
 fn render_mermaid_clipboard_html(source: &str) -> Option<String> {
-    let svg = mermaid_to_svg::render_mermaid_to_svg(source, Some(&MermaidTheme::light()))
+    let source = strip_mermaid_frontmatter(source);
+    let svg = mermaid_to_svg::render_mermaid_to_svg(source.as_ref(), Some(&MermaidTheme::light()))
         .ok()?
         .into_bytes();
     Some(mermaid_image_html(&svg))

crates/editor/src/content/mermaid_diagram.rs

@@ -1,4 +1,5 @@
 use std::{
+    borrow::Cow,
     hash::{DefaultHasher, Hash, Hasher},
     sync::Arc,
 };
@@ -24,8 +25,56 @@ struct MermaidDiagramAsset;
 
 impl AsyncAssetType for MermaidDiagramAsset {}
 
+/// Strip a leading Mermaid YAML frontmatter block (delimited by `---` lines
+/// on their own) from `source`, leaving the diagram body untouched when no
+/// frontmatter is present.
+///
+/// Mermaid 11 supports a `---\nconfig:\n  ...\n---` block at the top of a
+/// diagram for per-diagram configuration. The pinned `mermaid_to_svg`
+/// renderer's diagram-type detection treats the first non-empty,
+/// non-`%%`-prefixed line as the diagram token; with frontmatter that token
+/// becomes `---` instead of the actual diagram type (e.g. `xychart-beta`),
+/// and the renderer fails to dispatch to the right parser. Warp passes a
+/// hardcoded [`MermaidTheme::light`] anyway and does not honor any of the
+/// frontmatter config keys, so stripping is lossless. See
+/// warpdotdev/warp#10676.
+pub fn strip_mermaid_frontmatter(source: &str) -> Cow<'_, str> {
+    fn next_line_end(s: &str, start: usize) -> usize {
+        s[start..]
+            .find('\n')
+            .map(|p| start + p + 1)
+            .unwrap_or(s.len())
+    }
+
+    let mut cursor = 0;
+    while cursor < source.len() {
+        let end = next_line_end(source, cursor);
+        let line = source[cursor..end].trim();
+        if line.is_empty() {
+            cursor = end;
+            continue;
+        }
+        if line != "---" {
+            return Cow::Borrowed(source);
+        }
+        let mut scan = end;
+        while scan < source.len() {
+            let scan_end = next_line_end(source, scan);
+            if source[scan..scan_end].trim() == "---" {
+                return Cow::Owned(source[scan_end..].to_string());
+            }
+            scan = scan_end;
+        }
+        // Unterminated frontmatter — leave the source so the renderer's own
+        // error surfaces rather than silently dropping content.
+        return Cow::Borrowed(source);
+    }
+
+    Cow::Borrowed(source)
+}
+
 pub fn mermaid_asset_source(source: &str) -> AssetSource {
-    let source = source.to_string();
+    let source = strip_mermaid_frontmatter(source).into_owned();
     let mut hasher = DefaultHasher::new();
     source.hash(&mut hasher);
     let id = format!("light:{:x}", hasher.finish());

crates/editor/src/content/mermaid_diagram_tests.rs

@@ -1,3 +1,5 @@
+use std::borrow::Cow;
+
 use super::*;
 use crate::render::{layout::TextLayout, model::test_utils::TEST_STYLES};
 use warpui::{
@@ -37,6 +39,136 @@ fn loading_mermaid_layout_uses_default_height() {
     })
 }
 
+#[test]
+fn strip_frontmatter_removes_leading_config_block() {
+    // The exact sample from issue #10676.
+    let source = "---\nconfig:\n  theme: default\n---\nxychart-beta\n  title \"x\"\n";
+    let stripped = strip_mermaid_frontmatter(source);
+    assert_eq!(stripped, "xychart-beta\n  title \"x\"\n");
+    assert!(matches!(stripped, Cow::Owned(_)));
+}
+
+#[test]
+fn strip_frontmatter_preserves_source_without_frontmatter() {
+    let source = "graph TD\nA --> B\n";
+    let stripped = strip_mermaid_frontmatter(source);
+    assert_eq!(stripped, source);
+    assert!(
+        matches!(stripped, Cow::Borrowed(_)),
+        "no-op stripping should return Borrowed to avoid allocation",
+    );
+}
+
+#[test]
+fn strip_frontmatter_skips_leading_blank_lines_before_open_delimiter() {
+    let source = "\n\n   \n---\nconfig:\n  theme: dark\n---\npie\n  \"a\" : 1\n";
+    let stripped = strip_mermaid_frontmatter(source);
+    assert_eq!(stripped, "pie\n  \"a\" : 1\n");
+}
+
+#[test]
+fn strip_frontmatter_handles_crlf_line_endings() {
+    let source = "---\r\nconfig:\r\n  theme: default\r\n---\r\nflowchart TD\r\nA --> B\r\n";
+    let stripped = strip_mermaid_frontmatter(source);
+    assert_eq!(stripped, "flowchart TD\r\nA --> B\r\n");
+}
+
+#[test]
+fn strip_frontmatter_leaves_text_starting_with_three_dashes_then_content() {
+    // `--- something` (with content after the dashes) is not a frontmatter
+    // delimiter — the trimmed line is `--- something`, not `---`.
+    let source = "--- some weird title\nflowchart TD\nA --> B\n";
+    let stripped = strip_mermaid_frontmatter(source);
+    assert_eq!(stripped, source);
+    assert!(matches!(stripped, Cow::Borrowed(_)));
+}
+
+#[test]
+fn strip_frontmatter_leaves_unterminated_block_for_renderer_to_surface() {
+    // No closing `---`: leave intact so mermaid_to_svg can surface its own
+    // error instead of silently dropping the body.
+    let source = "---\nconfig:\n  theme: default\nflowchart TD\nA --> B\n";
+    let stripped = strip_mermaid_frontmatter(source);
+    assert_eq!(stripped, source);
+    assert!(matches!(stripped, Cow::Borrowed(_)));
+}
+
+#[test]
+fn strip_frontmatter_handles_empty_input() {
+    assert_eq!(strip_mermaid_frontmatter(""), "");
+}
+
+#[test]
+fn strip_frontmatter_handles_only_frontmatter_no_body() {
+    let source = "---\nconfig: {}\n---\n";
+    let stripped = strip_mermaid_frontmatter(source);
+    assert_eq!(stripped, "");
+}
+
+#[test]
+fn strip_frontmatter_handles_frontmatter_without_trailing_newline() {
+    // Closing `---` on the final line (no newline after).
+    let source = "---\nfoo: bar\n---";
+    let stripped = strip_mermaid_frontmatter(source);
+    assert_eq!(stripped, "");
+}
+
+#[test]
+fn strip_frontmatter_handles_open_delimiter_only_with_newline() {
+    // `---\n` and nothing else: treated as unterminated, leave intact.
+    let source = "---\n";
+    let stripped = strip_mermaid_frontmatter(source);
+    assert_eq!(stripped, source);
+    assert!(matches!(stripped, Cow::Borrowed(_)));
+}
+
+#[test]
+fn strip_frontmatter_handles_open_delimiter_only_without_newline() {
+    // Single line `---` with no newline at all: must not panic / loop forever;
+    // returned as-is for the renderer to surface.
+    let source = "---";
+    let stripped = strip_mermaid_frontmatter(source);
+    assert_eq!(stripped, source);
+    assert!(matches!(stripped, Cow::Borrowed(_)));
+}
+
+#[test]
+fn strip_frontmatter_treats_indented_dashes_as_delimiter() {
+    // Documents the intentional choice: a leading-whitespace `---` line is
+    // trimmed before comparison and treated as a frontmatter delimiter. This
+    // mirrors the renderer's own `first_diagram_type_token`, which trims each
+    // line before matching the diagram token — being stricter here would
+    // diverge from the renderer and leave a class of broken sources broken.
+    let source = "\t---\n  config: x\n  ---\nflowchart TD\nA --> B\n";
+    let stripped = strip_mermaid_frontmatter(source);
+    assert_eq!(stripped, "flowchart TD\nA --> B\n");
+}
+
+#[test]
+fn strip_frontmatter_does_not_treat_inner_dashes_line_as_delimiter() {
+    // The `---` between `title` and `body` here is NOT a frontmatter open
+    // because the first content line is `flowchart TD`, not `---`.
+    let source = "flowchart TD\n---\nA --> B\n";
+    let stripped = strip_mermaid_frontmatter(source);
+    assert_eq!(stripped, source);
+}
+
+#[test]
+fn mermaid_asset_source_hashes_post_strip_for_cache_key_stability() {
+    // The asset cache key must be derived from the post-strip source so that
+    // the same logical diagram with and without frontmatter doesn't churn
+    // the cache (and so the bug-fix actually changes what gets rendered).
+    let with_frontmatter = "---\nconfig:\n  theme: default\n---\nflowchart TD\nA --> B\n";
+    let without_frontmatter = "flowchart TD\nA --> B\n";
+    let with = mermaid_asset_source(with_frontmatter);
+    let without = mermaid_asset_source(without_frontmatter);
+    let (with_id, without_id) = match (&with, &without) {
+        (AssetSource::Async { id: a, .. }, AssetSource::Async { id: b, .. }) => (a, b),
+        _ => panic!("expected Async asset sources"),
+    };
+    assert_eq!(with_id, without_id);
+}
+
 #[test]
 fn failed_mermaid_layout_uses_compact_height() {
     App::test((), |app| async move {