fix: css animation forwards causing UI lag

[luwes]

when turning off hardware acceleration on Chrome it was reported that the Mux player UI became laggy.

I found one other user on Stackoverflow with the same issue:
https://stackoverflow.com/questions/12991164/maintaining-the-final-state-at-end-of-a-css-animation#comment118487774_12991203

Seems like part of the explanation is found here
https://developer.mozilla.org/en-US/docs/Web/CSS/animation-fill-mode#forwards

Animated properties behave as if included in a set will-change property value. If a new stacking context was created during the animation, the target element retains the stacking context after the animation has finished.

[vercel]

@luwes is attempting to deploy a commit to the Mux Team on Vercel.

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[vercel]

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