1. Use a constant for the classname instead of being hardcoded.
2. Use existing 'debounce' function instead of introducing another.
3. Added changelog entry.
Fixes #-4021.
## Changes:
When the autoplay overlay gets partially off-screen, the timer will be stopped. We can tweak how much down it needs to be scrolled.
When timer is paused, the countdown resets.
The pausing mechanism has no effect on the floating player (since the 'top' can never go negative). This behavior is the same as another big video platform.
