Problem

Scroll-driven animations (parallax effects, scroll progress bars, reveal animations) cannot be tested in jsdom environments. Without ScrollTimeline and ViewTimeline APIs, developers cannot test whether:

• Animations trigger at correct scroll positions
• Animation progress matches scroll position correctly
• Performance optimizations work (passive listeners, will-change)
• CSS animation-timeline property functions properly
• Modern scroll-based UI patterns behave as expected

Impact

• Most developers don't test scroll animations at all
• Manual scroll event calculations are complex and error-prone
• Modern UI patterns using scroll-driven animations go untested
• Performance regressions in scroll animations are not caught

Proposed Solution

Implement ScrollTimeline and ViewTimeline mocks that:

• Integrate with existing Web Animations API implementation
• Calculate timeline progress based on scroll position
• Handle ViewTimeline intersection logic with elements
• Support both JavaScript and CSS-based scroll animations
• Manage timeline state across multiple animations

Implementation Challenges

• Complex timeline calculation based on scroll position
• ViewTimeline requires intersection logic with elements
• Must integrate seamlessly with existing Web Animations API
• Need to handle both JS and CSS-based scroll animations
• Timeline state management across multiple animations

Current Implementation Status

• ❌ jsdom: No Web Animations API support at all
• ❌ happy-dom: Has basic Animation API but no timeline support
• ❌ linkedom: No implementation
• 🔧 Polyfill: flackr/scroll-timeline exists but is large

See future-features/scroll-timeline.md for detailed analysis.