fix(section-splitter): track code fence state to prevent false heading detection

Lines like `# vulcan.yaml` in YAML code blocks or `# Generate events` in shell code blocks were matching the heading pattern `/^(#{1,6})\s+(.+)$/`, causing the section splitter to create section boundaries inside code blocks. This resulted in code block content rendering outside the fence, with only language badges showing.

The fix implements proper fence tracking:
- Detects fence start/end with `/^(\`{3,}|~{3,})/`
- Tracks fence character (` or ~) and minimum length
- Enforces CommonMark closing rules (same char, >= opening length)
- Conditionally checks headings only when `!inCodeFence`

Author: jamesainslie

src/utils/section-splitter.ts

@@ -19,17 +19,44 @@ export interface MarkdownSection {
 export function splitIntoSections(markdown: string): MarkdownSection[] {
   const lines = markdown.split('\n');
   const sections: MarkdownSection[] = [];
-  
+
   let currentSection: string[] = [];
   let currentHeading: string | undefined;
   let currentLevel: number | undefined;
   let sectionStartLine = 0;
   let sectionId = 0;
-  
+
+  // Track fenced code block state
+  let inCodeFence = false;
+  let codeFenceChar = ''; // '`' or '~'
+  let codeFenceLength = 0; // minimum 3
+
   lines.forEach((line, index) => {
-    // Check if line is a heading (ATX style: # Heading)
-    const headingMatch = line.match(/^(#{1,6})\s+(.+)$/);
-    
+    // Check for code fence start/end (``` or ~~~)
+    const fenceMatch = line.match(/^(`{3,}|~{3,})/);
+
+    if (fenceMatch) {
+      const fenceStr = fenceMatch[1];
+      const fenceChar = fenceStr[0];
+      const fenceLen = fenceStr.length;
+
+      if (!inCodeFence) {
+        // Starting a code fence
+        inCodeFence = true;
+        codeFenceChar = fenceChar;
+        codeFenceLength = fenceLen;
+      } else if (fenceChar === codeFenceChar && fenceLen >= codeFenceLength) {
+        // Closing the code fence (must use same char and >= length)
+        inCodeFence = false;
+        codeFenceChar = '';
+        codeFenceLength = 0;
+      }
+      // If different fence char or shorter length, it's content inside the fence
+    }
+
+    // Only check for headings when NOT inside a code fence
+    const headingMatch = !inCodeFence && line.match(/^(#{1,6})\s+(.+)$/);
+
     if (headingMatch) {
       // Save previous section if it exists
       if (currentSection.length > 0) {
@@ -42,7 +69,7 @@ export function splitIntoSections(markdown: string): MarkdownSection[] {
           id: `section-${sectionId++}`,
         });
       }
-      
+
       // Start new section
       currentSection = [line];
       currentHeading = headingMatch[2];
@@ -52,7 +79,7 @@ export function splitIntoSections(markdown: string): MarkdownSection[] {
       currentSection.push(line);
     }
   });
-  
+
   // Add final section
   if (currentSection.length > 0) {
     sections.push({
@@ -64,7 +91,7 @@ export function splitIntoSections(markdown: string): MarkdownSection[] {
       id: `section-${sectionId++}`,
     });
   }
-  
+
   return sections;
 }
 
@@ -74,16 +101,16 @@ export function splitIntoSections(markdown: string): MarkdownSection[] {
 export function splitIntoChunks(markdown: string, chunkSize = 50000): MarkdownSection[] {
   const sections: MarkdownSection[] = [];
   const lines = markdown.split('\n');
-  
+
   let currentChunk: string[] = [];
   let currentSize = 0;
   let chunkStartLine = 0;
   let chunkId = 0;
-  
+
   lines.forEach((line, index) => {
     currentChunk.push(line);
     currentSize += line.length + 1; // +1 for newline
-    
+
     // If chunk is large enough, save it
     if (currentSize >= chunkSize) {
       sections.push({
@@ -92,13 +119,13 @@ export function splitIntoChunks(markdown: string, chunkSize = 50000): MarkdownSe
         endLine: index,
         id: `chunk-${chunkId++}`,
       });
-      
+
       currentChunk = [];
       currentSize = 0;
       chunkStartLine = index + 1;
     }
   });
-  
+
   // Add remaining lines
   if (currentChunk.length > 0) {
     sections.push({
@@ -108,7 +135,7 @@ export function splitIntoChunks(markdown: string, chunkSize = 50000): MarkdownSe
       id: `chunk-${chunkId++}`,
     });
   }
-  
+
   return sections;
 }
 
@@ -120,33 +147,32 @@ export function getInitialSections(
   options: { maxSections?: number; maxSize?: number; upToSectionId?: string } = {}
 ): MarkdownSection[] {
   const { maxSections = 3, maxSize = 30000, upToSectionId } = options;
-  
+
   // If we need to render up to a specific section (for scroll restoration),
   // render all sections up to and including that one
   if (upToSectionId) {
-    const targetIndex = sections.findIndex(s => s.id === upToSectionId);
+    const targetIndex = sections.findIndex((s) => s.id === upToSectionId);
     if (targetIndex !== -1) {
       // Render all sections up to the target, plus one more for context
       return sections.slice(0, Math.min(targetIndex + 2, sections.length));
     }
   }
-  
+
   const initial: MarkdownSection[] = [];
   let totalSize = 0;
-  
+
   for (const section of sections) {
     if (initial.length >= maxSections) break;
     if (totalSize + section.markdown.length > maxSize) break;
-    
+
     initial.push(section);
     totalSize += section.markdown.length;
   }
-  
+
   // Always include at least one section
   if (initial.length === 0 && sections.length > 0) {
     initial.push(sections[0]);
   }
-  
+
   return initial;
 }
-