code: function computeMirrorDimensions({source, ...args}) {
    return withPromise((resolve) => {
      const sourceRect = source.getBoundingClientRect();
      resolve({source, sourceRect, ...args});
    });
  }
docs: Computes mirror dimensions based on the source element
  Adds sourceRect to state
  @param {Object} state
  @param {HTMLElement} state.source
  @return {Promise}
  @private