calcTopPositionMode() {
        let menuStyle = window.getComputedStyle(this.openMenuNode);

        switch (this.positionMode.top) {
            case 'target':
                this.position.top = this.originNodeRect.top - this.alignTarget.offsetTop;
                break;
            case 'cascade':
                this.position.top = this.originNodeRect.top - parseFloat(menuStyle.paddingTop) - this.originNode.style.top;
                break;
            case 'bottom':
                this.position.top = this.originNodeRect.top + this.originNodeRect.height;
                break;
            default:
                assert(`Invalid target mode '${this.positionMode.top}' specified for paper-menu on Y axis.`);
        }
    }
Calcs top position based on the top position mode