_onmousedown(e) {
      // If step is 0 no interaction is available because there is no constant
      // (equal for all user environments) quantitative representation of the value
      if (this.disabled || this._effectiveStep === 0) {
        return;
      }
      // Calculate the new value from the press position of the event
      const newValue = this.handleDownBase(e);
      // Determine the rest of the needed details from the start of the interaction.
      this._saveInteractionStartData(e, newValue);
      this.rangePressed = this._isPressInCurrentRange;
      // Do not yet update the RangeSlider if press is in range or over a handle.
      if (this._isPressInCurrentRange || this._handeIsPressed) {
        this._handeIsPressed = false;
        return;
      }
      // Update Slider UI and internal state
      this.update(this._valueAffected, newValue, undefined);
    }
Called when the user starts interacting with the slider

@private