I found a problem with the positioning of popups in story maps (I tried it only with the series story maps). Take the ESRI example:
If I click on an entry on the far left, I get a popup positioned just to the right of the point. If I then click on a point on the far right, I get a popup positioned to the left of the point, off of the browser window. Then I click back on the left side point and the popup is now position to the left of the leftmost point. I've looked at the logic and it is positioning the popup based on the last point clicked and not the current point being clicked on.
Below are some screenshots of the above sequence.