While developing my personal website, I noticed that my mobile menu appeared for a split second and then moved out of the screen.
My mobile menu is placed on the right side of the screen by using the following CSS:

.navigation {
  transform: translateX(100%)
}

To make the menu come into the screen smoothly, I used the following CSS:

.navigation {
  transition-property: transform;
  transition-timing-function: cubic-bezier(0.4, 0, 0.2, 1);
  transition-duration: 150ms;
}

By adding this transition in my initial CSS, the menu will be transformed to the right side of the screen but with a duration of 150ms, so this will be clearly visible when the page loads.
To avoid this behaviour, I had to find a way to only apply the transition once the React app is already mounted.
For this use case, I found a custom useIsMounted hook useful.

import { useRef, useEffect } from 'react';

const useIsMounted = () => {
  const isMounted = useRef(false);
  useEffect(() => {
    isMounted.current = true;
    return () => isMounted.current = false;
  }, []);
  return isMounted;
};

export default useIsMounted;

Then in my component, I can easily import this hook and use it:

const Navigation = () => {
  const isMounted = useIsMounted();
  return (<div className={`navigation ${isMounted ? 'mounted' : ''}`}></div>)
}

My CSS then becomes this:

.navigation.mounted {
  transition-property: transform;
  transition-timing-function: cubic-bezier(0.4, 0, 0.2, 1);
  transition-duration: 150ms;
}

And that's it! No more unwanted animations on page load.
This will also help to increase your Cumulative Layout Shift (https://web.dev/cls) if you were encountering the same thing as I was, because your layout will not shift anymore, so it's a win-win!
You can see the end result here: https://thomasledoux.be (I use Tailwind on my site, so the example code is not exactly the same).