Issue with sidebar bottom margin when a pop-up appears

HI all— I have create a website ( with a sideBar that takes up 25 vw and main Div back taking up 75 vw. There are elements on the DIV that when clicked a pop-up appears.

However, if I scroll too far down and click on one of these elements, the bottom margin of the sidebar changes. It only works correctly if I do NOT scroll on the main Div at all. Anyone know how I go about fixing this? Is it an overflow issue?

Thank you kindly

Here is my site Read-Only: