_setAffectedValueByFocusedElement() {
      if (this.shadowRoot.activeElement === this._startHandle) {
        this._setAffectedValue("startValue");
      }
      if (this.shadowRoot.activeElement === this._endHandle) {
        this._setAffectedValue("endValue");
      }
      if (this.shadowRoot.activeElement === this._progressBar) {
        this._setAffectedValue(undefined);
      }
      this._setIsPressInCurrentRange(!this._valueAffected);
    }
Determines affected value (start/end) depending on the currently
active inner element within the Range Slider - used in the keyboard handling.

@private