catstodon/app/javascript/mastodon/features/ui/util/reduced_motion.js

// Like react-motion's Motion, but reduces all animations to cross-fades
// for the benefit of users with motion sickness.
import React from 'react';
import Motion from 'react-motion/lib/Motion';
import PropTypes from 'prop-types';

const stylesToKeep = ['opacity', 'backgroundOpacity'];

const extractValue = (value) => {
  // This is either an object with a "val" property or it's a number
  return (typeof value === 'object' && value && 'val' in value) ? value.val : value;
};

class ReducedMotion extends React.Component {

  static propTypes = {
    defaultStyle: PropTypes.object,
    style: PropTypes.object,
    children: PropTypes.func,
  }

  render() {

    const { style, defaultStyle, children } = this.props;

    Object.keys(style).forEach(key => {
      if (stylesToKeep.includes(key)) {
        return;
      }
      // If it's setting an x or height or scale or some other value, we need
      // to preserve the end-state value without actually animating it
      style[key] = defaultStyle[key] = extractValue(style[key]);
    });

    return (
      <Motion style={style} defaultStyle={defaultStyle}>
        {children}
      </Motion>
    );
  }

}

export default ReducedMotion;