fix: robustify initial scroll value detection when scroll is smooth

[dummdidumm]

• the previous assumption was wrong: browser don't fire a scroll event initially when the scroll isn't smooth
• the previous logic wasn't using the "is scrolling now" logic which meant the render effect fired immediately after, causing smooth scrolling to start too late to be overridden

adjusted the comment and reused the scroll handler function to guard against the race condition
fixes #11623

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