How to reproduce: While the game window is minimized, go to multiplayer screen (singleplayer screen might be affected too), have a lot of items in there, scroll all the way down, maximize the window (putting on full screen might have the same effect) and voila! A interesting scrollbar!
Video shows the bug in action.
Thank you for your report!
We're actually already tracking this issue in MC-158271, so I resolved and linked this ticket as a duplicate.
If you would like to add a vote and any extra information to the main ticket it would be appreciated.
If you haven't already, you might like to make use of the search feature in the future to see if the issue has already been reported.
Quick Links:
📓 Issue Guidelines – 💬 Community Support – 📧 Customer Support – 📖 Game Wiki