fix: Remove reliance on accidental behavior

[dkennell]

The initial call of tick ought to have a value for the 'now' argument' passed into it here. Currently, the code does work incidentally, but it passes undefined into tick, with the result that ref.current.style.opacity gets set equal to NaN. The browser just ignores this, so nothing bad happens but it's fragile and could be confusing to readers. I suggest we pass startTime to tick when it's first called, so that ref.current.style.opacity just gets set equal to 0 instead.

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[github-actions]

Size changes

📦 Next.js Bundle Analysis for react-dev

This analysis was generated by the Next.js Bundle Analysis action. 🤖

This PR introduced no changes to the JavaScript bundle! 🙌

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