instrumental/app/javascript/mastodon/components/modal_root.js

import React from 'react';
import PropTypes from 'prop-types';
import 'wicg-inert';

export default class ModalRoot extends React.PureComponent {

  static propTypes = {
    children: PropTypes.node,
    onClose: PropTypes.func.isRequired,
  };

  state = {
    revealed: !!this.props.children,
  };

  activeElement = this.state.revealed ? document.activeElement : null;

  handleKeyUp = (e) => {
    if ((e.key === 'Escape' || e.key === 'Esc' || e.keyCode === 27)
         && !!this.props.children) {
      this.props.onClose();
    }
  }

  handleKeyDown = (e) => {
    if (e.key === 'Tab') {
      const focusable = Array.from(this.node.querySelectorAll('button:not([disabled]), [href], input:not([disabled]), select:not([disabled]), textarea:not([disabled]), [tabindex]:not([tabindex="-1"])')).filter((x) => window.getComputedStyle(x).display !== 'none');
      const index = focusable.indexOf(e.target);

      let element;

      if (e.shiftKey) {
        element = focusable[index - 1] || focusable[focusable.length - 1];
      } else {
        element = focusable[index + 1] || focusable[0];
      }

      if (element) {
        element.focus();
        e.stopPropagation();
        e.preventDefault();
      }
    }
  }

  componentDidMount () {
    window.addEventListener('keyup', this.handleKeyUp, false);
    window.addEventListener('keydown', this.handleKeyDown, false);
  }

  componentWillReceiveProps (nextProps) {
    if (!!nextProps.children && !this.props.children) {
      this.activeElement = document.activeElement;

      this.getSiblings().forEach(sibling => sibling.setAttribute('inert', true));
    } else if (!nextProps.children) {
      this.setState({ revealed: false });
    }
    if (!nextProps.children && !!this.props.children) {
      this.activeElement.focus();
      this.activeElement = null;
    }
  }

  componentDidUpdate (prevProps) {
    if (!this.props.children && !!prevProps.children) {
      this.getSiblings().forEach(sibling => sibling.removeAttribute('inert'));
    }
    if (this.props.children) {
      requestAnimationFrame(() => {
        this.setState({ revealed: true });
      });
    }
  }

  componentWillUnmount () {
    window.removeEventListener('keyup', this.handleKeyUp);
    window.removeEventListener('keydown', this.handleKeyDown);
  }

  getSiblings = () => {
    return Array(...this.node.parentElement.childNodes).filter(node => node !== this.node);
  }

  setRef = ref => {
    this.node = ref;
  }

  render () {
    const { children, onClose } = this.props;
    const { revealed } = this.state;
    const visible = !!children;

    if (!visible) {
      return (
        <div className='modal-root' ref={this.setRef} style={{ opacity: 0 }} />
      );
    }

    return (
      <div className='modal-root' ref={this.setRef} style={{ opacity: revealed ? 1 : 0 }}>
        <div style={{ pointerEvents: visible ? 'auto' : 'none' }}>
          <div role='presentation' className='modal-root__overlay' onClick={onClose} />
          <div role='dialog' className='modal-root__container'>{children}</div>
        </div>
      </div>
    );
  }

}