JS Tooltip Positioning Flicker

Question

Writing a pure JS tooltip library and have made it work really, really well. No big bells and whistles, just a super lightweight super utilitarian easy-to-customize tooltip. That said, I'm having an issue with updating the position; specifically when the tooltip is detected to be out of bounds.

And it works! It does. Issue is that when I'm hovered over the target element, which is against the right side of the page which causes the tooltip to be out of bounds, it will flicker between the default bottom-right position and the correct bottom-left position. Upon further inspection, it seems to only register out of bounds on every other hair I move the cursor. For instance, when I enter the element, it will display properly; move the cursor a hair in any direction and it will seemingly reset to bottom-right, not seeing that it's out of bounds. Move it one hair further and it will figure out it's out of bounds again, correcting itself to bottom-left. I'm not sure what assumptions to make here, but I'll let you guys be the judge of it.

    updateTooltip(event) {
        const mouseX = event.pageX,
            mouseY = event.pageY;

        const adjustment = 15;

        let position = (this.element.getAttribute(this.selector + '-pos') || '').split(' ');

        if (position.length !== 2) position = [ 'bottom', 'right' ];

        var isOut = this.checkBounds();
        if (isOut.top)    { position[0] = 'bottom'; }
        if (isOut.left)   { position[1] = 'right'; }
        if (isOut.bottom) { position[0] = 'top'; }
        if (isOut.right)  { position[1] = 'left'; }

        let vertical, horizontal;

        switch (position[0]) {
            case 'top':
                vertical = -adjustment - this.tooltip.offsetHeight;
                break;
            case 'center':
                vertical = 0 - (this.tooltip.offsetHeight / 2);
                break;
            default:
                vertical = adjustment;
        }

        switch (position[1]) {
            case 'left':
                horizontal = -adjustment - this.tooltip.offsetWidth;
                break;
            case 'center':
                horizontal = 0 - (this.tooltip.offsetWidth / 2);
                break;
            default:
                horizontal = adjustment;
        }

        this.tooltip.style.top  = mouseY + vertical   + 'px';
        this.tooltip.style.left = mouseX + horizontal + 'px';
    }

    checkBounds() {
        if (!this.tooltip) return;

        const bounds = this.tooltip.getBoundingClientRect();

        let out = {};
        out.top = bounds.top < 0;
        out.left = bounds.left < 0;
        out.bottom = bounds.bottom > (window.innerHeight || document.documentElement.clientHeight);
        out.right = bounds.right > (window.innerWidth || document.documentElement.clientWidth);
        out.any = out.top || out.left || out.bottom || out.right;
        out.all = out.top && out.left && out.bottom && out.right;

        return out;
    }