The Windows 10 taskbar does not scroll in when UnrealEd is open and in fullscreen mode. Windows has this feature in all recent versions, and it creates a bit of extra vertical screen real estate. Usually, I can then hover the cursor over the bottom of the screen and the taskbar opens up. But this doesn't work when UnrealEd is in fullscreen mode. Instead, I have to press the Windows key to see the taskbar.
In Windows 10, open Taskbar Settings.
Enable the option "Automatically hide the taskbar in desktop mode".
Open UnrealEd. Maximize it, setting it to fullscreen mode.
Hover over the bottom of the screen.
Observe that the taskbar does not scroll in when the mouse hovers over the area, as it does with other Windows programs.
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-57821 in the post.