In a recent update (within the last couple months), WordPress added the ability to enable full-height editor and distraction-free functionality within a page/post. If you’re in a page or post, click on your Screen Options and see if the box is checked or not.
Enabling the full-height editor allows you to use that far right scroller and includes the content box instead of “confining” the content box and creating its own “scroller” as the content gets longer.
When you disable the full-height editor, the scroller will appear within the content box as well as the far right.
Hope this helps! I had clients ask about something being different…and it does take some getting used to. I find myself switching back and forth still. Sometimes I “want” that extra scroller in the content box.