Hi tullio0106,
When I tested the tooltip with the code that you have provided the tooltip appears truncated/cut when it overflowing the browser window which is expected, but when setting the position to absolute I was able to access the checkboxes for the dropdown.
However for your scenario I would suggest to set the tooltip position to absolute and add the following code snippet so that the tooltip is closed when the dropdown is opened:

 const dropDownList = document.querySelector('smart-drop-down-list'),
                    tooltip  = document.querySelector('smart-tooltip');
            dropDownList.addEventListener('open', () => {
                if(dropDownList.opened) {

Please, do not hesitate to contact us if you have any additional questions.
Best regards,
Yavor Dashev
Smart UI Team