Implement scroll throttling in HTMLDocument::simulateScrollWithOffset for performance optimization

[Copilot]

Problem

The HTMLDocument::simulateScrollWithOffset method was dispatching scroll events on every scroll operation without any rate limiting, causing performance issues during rapid scrolling scenarios. This was inconsistent with the Element::simulateScrollWithOffset method, which already had throttling implemented.

Solution

Added 16ms scroll event throttling (~60fps) to HTMLDocument::simulateScrollWithOffset to match the existing implementation in Element::simulateScrollWithOffset. This ensures consistent performance optimization across the entire DOM hierarchy.

Changes Made

src/client/dom/document.hpp:

• Added #include <chrono> for time-based throttling
• Added private shouldThrottleScrollEvent() method declaration
• Added throttling fields:
  • last_scroll_event_time_ to track the last scroll event time
  • scroll_throttle_duration_{16} for 60fps throttling (16ms intervals)

src/client/dom/document.cpp:

• Implemented shouldThrottleScrollEvent() method using the same logic as Element
• Modified simulateScrollWithOffset() to use throttling before dispatching scroll events
• Only dispatches scroll events when the throttle duration has elapsed

docs/internals/DOM_SCROLLING_PERFORMANCE.md:

• Updated documentation to reflect that both Element and HTMLDocument now have throttling
• Added implementation details for document-level scrolling optimization

Performance Impact

• Before: Unlimited scroll event frequency during rapid scrolling
• After: Maximum 60fps scroll event rate (16ms throttling)
• Benefit: Significant reduction in CPU overhead and improved scroll performance consistency

The implementation is identical to the existing Element throttling mechanism, ensuring maintainability and consistent behavior across the codebase.

Testing

• ✅ C++ formatting check passes
• ✅ Code compiles without errors
• ✅ Implementation matches existing Element throttling pattern
• ✅ Documentation updated to reflect changes

This change maintains full backward compatibility while providing the performance optimization requested in the issue.

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