Hi. This bug I have noticed while making topics on the DevForum that happens when full screening the new topic popup that makes it appear in a way that causes the user the ability not to exit the full screen (unless you press the close button in the popup).
Steps to replicate
Click “+ New Topic” button in any category
Click the full screen icon
Congratulations, you replicated the bug!
EDIT
This bug also slightly hides the formatting buttons bar too.