To replicate, go to any article with an image and click the image. Then click full screen and exit by clicking on the X in the right hand corner.
Every time I do this it has some undesired effects. My scroll bar disappears and sometimes scrolling is completely disabled. I have tried this both on Mac and Windows 8, both have the same effect.
@Steakyfask1, I'm assuming that this is happening with Media Viewer in i.e. Wikipedia?
Associating this task with MediaViewer for now, as this task is currently orphan.
For MediaViewer, this is expected behavior (except the "sometimes" part - scrolling should always be disabled), if by fullscreen you mean activating the Fullscreen API via the icon below the close button.
Mass-removing the Multimedia tag from MediaViewer tasks, as this is now being worked on by the Reading department, not Editing's Multimedia team.
I fear the initial triage and investigation may have missed the point of this bug - @Steakyfask1 are you saying that, after exiting the Media Viewer, the scrollbar for the article is disabled? If so, can you please tell us what browser you're using? I could not reproduce this on English Wikipedia with Google Chrome on Linux.