chore: improve errors for bad YAML frontmatter

Adds a custom error message for any mermaid diagram that starts with
a `---`. Normally, these are expected to be part of a YAML front-matter
block, but indentation issues or a missing closing `---` may cause
these to be not parsed correctly.

packages/mermaid/src/diagram-api/diagram-orchestration.ts

@@ -125,6 +125,33 @@ export const addDiagrams = () => {
     },
     (text) => text.toLowerCase().trim() === 'error'
   );
+  registerDiagram(
+    '---',
+    // --- diagram type may appear if YAML front-matter is not parsed correctly
+    {
+      db: {
+        clear: () => {
+          // Quite ok, clear needs to be there for --- to work as a regular diagram
+        },
+      },
+      styles: errorStyles, // should never be used
+      renderer: errorRenderer, // should never be used
+      parser: {
+        parser: { yy: {} },
+        parse: () => {
+          throw new Error(
+            'Diagrams beginning with --- are not valid. ' +
+              'If you were trying to use a YAML front-matter, please ensure that ' +
+              "you've correctly opened and closed the YAML front-matter with unindented `---` blocks"
+          );
+        },
+      },
+      init: () => null, // no op
+    },
+    (text) => {
+      return text.toLowerCase().trimStart().startsWith('---');
+    }
+  );
 
   registerDiagram(
     'c4',

packages/mermaid/src/mermaidAPI.spec.ts

@@ -629,6 +629,19 @@ describe('mermaidAPI', function () {
       expect(mermaid.parseError).toEqual(undefined);
       expect(() => mermaidAPI.parse('this is not a mermaid diagram definition')).toThrow();
     });
+    it('throws for a nicer error for a invalid definition starting with `---`', function () {
+      expect(mermaid.parseError).toEqual(undefined);
+      expect(() =>
+        mermaidAPI.parse(`
+      ---
+      title: a malformed YAML front-matter
+      `)
+      ).toThrow(
+        'Diagrams beginning with --- are not valid. ' +
+          'If you were trying to use a YAML front-matter, please ensure that ' +
+          "you've correctly opened and closed the YAML front-matter with unindented `---` blocks"
+      );
+    });
     it('does not throw for a valid definition', function () {
       expect(() => mermaidAPI.parse('graph TD;A--x|text including URL space|B;')).not.toThrow();
     });

packages/mermaid/src/utils.spec.js

@@ -238,9 +238,9 @@ Alice->Bob: hi`;
     const type = detectType(str);
     expect(type).toBe('gitGraph');
   });
-  it('should not allow frontmatter with leading spaces', function () {
+  it('should handle malformed frontmatter (with leading spaces) with `---` error graphtype', function () {
     const str = '    ---\ntitle: foo\n---\n  gitGraph TB:\nbfs1:queue';
-    expect(() => detectType(str)).toThrow('No diagram type detected for text');
+    expect(detectType(str)).toBe('---');
   });
 });
 describe('when finding substring in array ', function () {