Improve smooth scroll behavior to prevent button flickering

[hexart]

Smooth Scroll Button Behavior Fix

This PR addresses the issue where the scroll button briefly reappears during smooth scrolling animation.

Problem

When using behavior: 'smooth' for scrolling, the scroll button would flicker due to handleScroll being triggered during the animation process, causing showScrollButton to be set to true before reaching the bottom.

Solution

1. Added an isScrolling state to track scroll animation status
2. Created a checkScrollEnd function at component level to monitor actual scroll position and detect when scrolling has completely stopped
   • Placing this function at component level improves performance by avoiding function recreation on each button click
3. Enhanced both scrollToBottom and scrollToPosition functions to properly handle smooth scrolling:
   • Setting isScrolling state to true at start
   • Hiding scroll button during animation
   • Using checkScrollEnd to detect end of animation
   • Updating button visibility based on final position

Edge Case Handling

The solution also handles user interaction during scroll animation. When users interrupt auto-scroll with mouse wheel, the scroll button visibility is immediately updated when scrolling stops, accurately reflecting the current position without requiring additional scroll input.