get focusTreeId() {
    return this._focusTreeIds[0];
  }
We maintain a separate instance variable to contain context that will be
passed down to descendants of this component so we can have a consistent
reference to the same object, rather than instantiating a new one on each
render, causing unnecessary re-rendering of descendant components that
consume the context.

@see https://reactjs.org/docs/context.html#caveats