A few users using the Windows 10 Insider Preview have found that after a certain point in the Prerelease builds, starting at 14915 it seems, the Unreal Editor and Epic Games Launcher context menus have stopped displaying correctly.
This is not editor version specific and only depends on being on one of these prerelease builds.
A user mentioned that this does not happen when running the editor with OpenGL. Another user found a line of code that can be changed to fix the issue, although I cannot say that this wouldn't break other things.
Inside of: Engine\Source\Runtime\Core\Private\Windows\WindowsWindow.cpp
Commenting out this line fixes the issue:
Head over to the existing AnswerHub thread and let us know what's up.