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Modal popup - restricting background scroll


I created a modal popup as per the instructions in the link below -

I have also used interactions to make the modal popup visible/invisible as desired. In the link above, the modal popup seems to work in a way wherein if the user scrolls down, the background scrolls with it. I tried setting the position to "Fixed" for the modal popup box but now the whole page scrolls down. I want to implement it in a way where the user can scroll within the modal popup but the background stays fixed. For example, go to and type "interactions" in the search box. The result gets displayed in a modal popup. When you scroll down, you will observe the background being fixed and the results within the modal popup scrolling down. Can you please help me accomplish this behavior? After trying different things, I am stuck in this issue.