fix: YouTube autoplay fails after track #400 (#3541)

[suryaprakash0010]

Fix: YouTube Autoplay Fails After Track 400

🎯 Issue Summary

Fixes #3541 - YouTube fails to autoplay next track for all tracks past 400 in large playlists.

🔍 Root Cause

The playlistUpNextAutoplay function was incorrectly setting currentIndex = totalVideos when playlist_up_next_autoplay was disabled, forcing playlists to end and breaking autoplay for large playlists beyond YouTube's ~200 video pagination limit.

🛠️ Solution

Core Fix

• Before: Forced playlist termination by setting currentIndex = totalVideos
• After: Proper synchronization between currentIndex and localCurrentIndex to maintain playlist continuity

Enhanced Features

• Large Playlist Handler: Monitors video changes and ensures proper index synchronization
• Observer Pattern: Detects playlist data updates when YouTube loads new segments
• Memory Management: Proper cleanup of observers and event listeners
• Navigation Support: Handles navigation between playlist and non-playlist pages

📁 Files Changed

📝 js&css\web-accessible\www.youtube.com\playlist.js

• Fixed playlistUpNextAutoplay() logic
• Added playlistLargePlaylistHandler() function
• Added cleanupPlaylistHandlers() function

📝 js&css\web-accessible\functions.js

• Integrated large playlist handler into videoPageUpdate()
• Added cleanup logic for non-playlist pages

📝 js&css\web-accessible\init.js

• Added handler to playlist initialization in yt-page-data-updated
• Added cleanup in yt-navigate-finish for navigation

🧪 test-large-playlist-fix.js (New)

• Comprehensive test script for verification
• Manual testing instructions

🔄 How It Works

1. YouTube's Limitation: Loads playlists in chunks (~200 videos)
2. Our Solution: Continuously monitors and synchronizes playlist indices
3. Result: Seamless autoplay beyond track 400

🧪 Testing

Automated Test

// Run in browser console on any YouTube playlist
testLargePlaylistFix();

Manual Testing

1. Find a playlist with 400+ videos
2. Start playing from video 200 or later
3. Enable/disable autoplay settings
4. Verify autoplay continues seamlessly

✅ Verification

• ✅ Fixes autoplay failure after track 400
• ✅ Maintains backward compatibility
• ✅ Proper memory management
• ✅ Handles navigation correctly
• ✅ Works with existing playlist features

📊 Impact

• Files Modified: 3 core files + 1 test file
• Lines Added: ~200 lines
• Breaking Changes: None
• Performance: Minimal impact

🎉 Result

Users can now autoplay through large playlists without interruption after track 400, resolving the long-standing issue reported in #3541.

[ImprovedTube]

thank you so much! @suryaprakash0010