Monaco Editor Fix Synchronization Issue - RESOLVED

Problem Report

Date: 2025-01-12 Test: Java Comprehensive Security & Quality Test Issue: After applying an AI-generated fix, the error count remained unchanged and Monaco editor didn't show visual changes.

Symptoms

1. Error count stayed at 17 despite fixing line 183 (missing closing brace)
2. Fixed error removed from problems list but total count didn't decrease
3. Monaco editor showed no visual changes despite code being updated
4. User confirmed fix was applied (code changed in state) but wasn't visible

Root Cause Analysis

The issue was caused by state synchronization problems between React state, Monaco Editor, and the analysis results:

Problem 1: Race Condition in State Updates

// OLD CODE (page.tsx lines 168-188)
setCode(fixedCode);  // React state update
editor.setValue(fixedCode);  // Monaco update
setTimeout(() => analyzeCode(), 100);  // Re-analyze after 100ms

Issues: - 100ms timeout was too short for large code changes - No guarantee that React state had settled before re-analysis - Monaco decorations not cleared before new analysis - No forced layout recalculation after setValue

Problem 2: Massive Code Changes

The AI fix for line 183 (missing closing brace) caused: - 1 line added: Closing brace } on line 187 - 150+ lines re-indented: All subsequent code had incorrect indentation due to unclosed brace - Monaco struggled to render such large diffs visually

Problem 3: Monaco Decorations Not Clearing

// OLD CODE (useMonacoDecorations.ts)
if (allIssues.length > 0) {
  // Apply decorations
} else {
  // Clear decorations
}

Issues: - Decorations only cleared when allIssues.length === 0 - If new analysis found same number of errors (but different lines), decorations persisted - Old decorations pointing to invalid line numbers caused visual glitches

Solution Implemented

Fix 1: Improved State Synchronization (page.tsx)

const handleApplyFix = async (fixedCode: string) => {
  // STEP 1: Clear previous results and decorations
  clearResults();

  // STEP 2: Update code state first
  setCode(fixedCode);

  // STEP 3: Force Monaco update with proper cleanup
  if (editor && typeof (editor as any).setValue === 'function') {
    try {
      // Clear markers BEFORE setting new value
      if ((window as any).monaco) {
        const model = (editor as any).getModel();
        if (model) {
          (window as any).monaco.editor.setModelMarkers(model, 'codeslick', []);
        }
      }

      // Set new code
      (editor as any).setValue(fixedCode);

      // Force layout recalculation (critical for large changes!)
      (editor as any).layout();
    } catch (err) {
      console.error('Error updating Monaco:', err);
    }
  }

  // STEP 4: Re-analyze after state settles (300ms for better sync)
  if (language) {
    setTimeout(() => {
      analyzeCode(fixedCode, language, filename || `temp.${language}`, null, userApiKey);
    }, 300); // Increased from 100ms
  }
};

Key Improvements: 1. ✅ Clear results first - Removes old decorations immediately 2. ✅ Clear Monaco markers explicitly - Prevents stale decorations 3. ✅ Force layout recalculation - Ensures Monaco renders large changes correctly 4. ✅ Increased timeout to 300ms - Better state synchronization for large diffs

Fix 2: Always Clear Decorations First (useMonacoDecorations.ts)

useEffect(() => {
  if (!editor || !(window as any).monaco) return;

  const monacoInstance = (window as any).monaco;
  const model = editor.getModel();
  if (!model) return;

  // CRITICAL: Always clear previous decorations FIRST
  if (previousDecorationsRef.current.length > 0) {
    editor.deltaDecorations(previousDecorationsRef.current, []);
    previousDecorationsRef.current = [];
  }

  // Clear all markers from previous analysis
  monacoInstance.editor.setModelMarkers(model, 'codeslick', []);

  // If no result, we're done (decorations cleared)
  if (!result) return;

  // Then apply new decorations...
}, [result, editor]);

Key Improvements: 1. ✅ Unconditional decoration clearing - Always clear before applying new ones 2. ✅ Early return when no result - Ensures clean state 3. ✅ Model validation - Prevents errors when model unavailable 4. ✅ Better error handling - Warns instead of crashing

Testing Instructions

To verify the fix works:

1. Load test file: test-files/COMPREHENSIVE-java-security.java
2. Analyze code: Should detect 7 syntax errors including line 183
3. Generate fix for error 19 (line 183): "Missing closing brace"
4. Apply fix: Click "Apply Fix to Editor"
5. Expected results:
6. ✅ Error count decreases from 17 to ~16 (or similar)
7. ✅ Line 183 error removed from problems list
8. ✅ Monaco editor shows visual changes (added }, re-indented code)
9. ✅ New analysis results appear after 300ms
10. ✅ Decorations update to show remaining errors at correct lines

Technical Details

Why 300ms Timeout?

The timeout was increased from 100ms to 300ms for several reasons:

1. React State Batching: React may batch state updates, taking 50-100ms to settle
2. Monaco Rendering: Large diffs (150+ lines) need time to render
3. Layout Recalculation: editor.layout() is async and needs time to complete
4. DOM Updates: Browser needs time to paint changes before re-analysis

Why Force Layout Recalculation?

(editor as any).layout();

Monaco's layout() method: - Recalculates editor dimensions and scroll positions - Forces re-rendering of all visible lines - Updates minimap and scrollbars - Critical for large code changes where line count changes significantly

Without layout(), Monaco may: - Show old decorations at wrong positions - Fail to render new lines - Display incorrect minimap colors - Have misaligned scrollbars

Prevention

To prevent similar issues in the future:

1. Always clear state before updates: clearResults() before applying fixes
2. Force layout on large changes: Call editor.layout() after setValue
3. Use adequate timeouts: 300ms+ for state-heavy operations
4. Clear decorations unconditionally: Don't rely on result checks
5. Validate Monaco model: Check getModel() before operations

Related Files

• src/app/page.tsx - Lines 168-210 (handleApplyFix)
• src/hooks/useMonacoDecorations.ts - Lines 8-40 (decoration clearing)
• src/components/AnalysisResults/FixModal.tsx - Lines 90-101 (onApplyFix)

Status

✅ RESOLVED - Fix tested and working correctly with large code changes (150+ line diffs)