I think I have found a bug with the scrollbar within the shortlist story map. I was able to replicate this bug within the San Diego Shortlist example.
Here are the steps to produce the behavior:
1) Click on Convention Center Skywalk to bring up its description.
2) Within the description panel, click the forward arrow to progress to the Gaslamp Quarter description.
3) Click the X to close out of the description panel.
4) Click on the Convention Center Skywalk again to bring up its description. Here, you'll notice that the scrollbar has disappeared. The only way to get it back is to click the forward arrow again to progress to the Gaslamp Quarter and then click the backward arrow to go back to the Convention Center Skywalk where the scrollbar will be visible again.
I tested this bug in both Google Chrome and Microsoft Edge.