Add detailed testing instructions for spam-click fix

- Explains why bug might not always reproduce (race condition)
- Provides step-by-step reproduction steps
- Shows exact code changes (only 2 small modifications)
- Includes multiple test cases

Author: gourijain026@gmail.com

TESTING_INSTRUCTIONS_6010.md

@@ -0,0 +1,124 @@
+# Testing Instructions for PR #6013 (Issue #6010)
+
+## Issue Summary
+Spam-clicking the Run button causes a crash with error:
+```
+Uncaught TypeError: Cannot read properties of undefined (reading 'unhighlight')
+at Blocks.unhighlight (blocks.js:2974)
+```
+
+## Changes Made
+This PR makes two minimal changes:
+
+### 1. `js/blocks.js` (Line 2973)
+**Before:**
+```javascript
+if (thisBlock !== null) {
+    this.blockList[thisBlock].unhighlight();
+}
+```
+
+**After:**
+```javascript
+if (thisBlock !== null && this.blockList[thisBlock]) {
+    this.blockList[thisBlock].unhighlight();
+}
+```
+**Why:** Adds defensive check to ensure block exists before calling unhighlight()
+
+### 2. `js/activity.js` (Line 1656)
+**Before:**
+```javascript
+this._doFastButton = env => {
+    this._onResize();
+    this.blocks.activeBlock = null;
+    hideDOMLabel();
+```
+
+**After:**
+```javascript
+this._doFastButton = env => {
+    // Prevent spam-clicking by checking if already running
+    if (this.logo._alreadyRunning) {
+        return;
+    }
+
+    this._onResize();
+    this.blocks.activeBlock = null;
+    hideDOMLabel();
+```
+**Why:** Prevents multiple simultaneous executions when Run button is clicked rapidly
+
+## How to Test
+
+### Setup
+1. Checkout this branch: `git checkout fix-spam-click-unhighlight-6010`
+2. Install dependencies: `npm install`
+3. Start the server: `node index.js`
+4. Open browser: `http://localhost:3000`
+
+### Test Case 1: Reproduce the Original Bug (Without Fix)
+To see the original issue, you need to create the right conditions:
+
+1. Create a simple program with multiple blocks:
+   - Add a **Start** block
+   - Add a **Repeat** block (set to 10 or more)
+   - Inside repeat, add **Note** block with **Pitch** block
+   
+2. Open browser console (F12 or Cmd+Option+I)
+
+3. **Rapidly click the Run button (red play button) 10-15 times very quickly**
+   - Click as fast as you can
+   - The key is to click before the first execution completes
+
+4. **Expected behavior WITHOUT fix:** 
+   - Console shows error: `Uncaught TypeError: Cannot read properties of undefined (reading 'unhighlight')`
+   - Execution may stop or behave erratically
+   - Block highlighting breaks
+
+### Test Case 2: Verify the Fix Works
+With this PR's changes:
+
+1. Use the same program from Test Case 1
+2. Open browser console
+3. **Rapidly click the Run button 10-15 times very quickly**
+4. **Expected behavior WITH fix:**
+   - No errors in console
+   - Program runs smoothly
+   - Only one execution happens (subsequent clicks are ignored while running)
+   - Block highlighting works correctly
+
+### Test Case 3: Normal Operation Still Works
+1. Create any simple program
+2. Click Run button once
+3. Wait for completion
+4. Click Run button again
+5. **Expected:** Normal execution, no issues
+
+### Test Case 4: Run Multiple Programs
+1. Create two separate Start blocks with different programs
+2. Click Run on first program
+3. Immediately click Run on second program
+4. **Expected:** First program completes, second program runs after
+
+## Why the Bug Might Not Always Reproduce
+
+The reviewer mentioned not seeing the error locally. This is because:
+
+1. **Timing-dependent:** The bug only occurs when clicks happen during a specific window while blocks are being highlighted/unhighlighted
+2. **Speed-dependent:** You need to click VERY fast (within milliseconds)
+3. **Program-dependent:** Longer-running programs (with loops/repeats) make it easier to reproduce
+4. **Race condition:** It's a classic race condition that doesn't always manifest
+
+## Verification
+Run the test suite to ensure no regressions:
+```bash
+npm test
+```
+
+All 3827 tests should pass (one playwright test may fail due to unrelated issue).
+
+## Additional Notes
+- The fix uses the existing `_alreadyRunning` flag that's already part of the codebase
+- Changes are minimal and defensive - they don't alter normal execution flow
+- Even if the bug is hard to reproduce, the defensive checks prevent potential crashes