Problem is: the total hight of the H1+IMG = 287.3 px, the maximum size of the inner content of the total div is 280px. So you've got 7.3px too much, which isn't hidden. So if the outer div can't resize, a scrollbar is the only solution for a browser as it has to show the content.
Solutions are either to hide the overflow, or change the maximum height of the outer div to be more than 20em, or set a maximum height for the image or find another trick to magnify the image and don't show the magnify border that's now present or to change the padding of the outer div.

Change 302262 had a related patch set uploaded (by Lucie Kaffee):
Make image and box both a corresponding px size

https://gerrit.wikimedia.org/r/302262

Change 302262 merged by jenkins-bot:
Make image and box both a corresponding px size

https://gerrit.wikimedia.org/r/302262