preact-context-provider.js

// - yes, this is the entire implementation
// - no, it doesn't actually depend on preact
export class Provider {
  getChildContext () {
    let { children, ...context } = this.props;
    return context;
  }
  render ({ children }) {
    return children && children[0] || null
  }
}

Good workg. :) Problem is the children prop will be passed as context as well.

export class Provider {
  getChildContext () {
    const contextsToProvide = { ...this.props }
    // remove not needed children prop for context
    delete contextsToProvide.children
    // all props are made available as context
    return { ...contextsToProvide }
  }
  render ({ children }) {
    return children && children[0] || null
  }
}

Instead of mutation, you could use destructuring to filter out any values you don't want without having to mutate:

export class Provider {
  getChildContext () {
    const { children, ...context } = this.props;
    return context;
  }
  render ({ children }) {
    return children && children[0] || null
  }
}

Amended!