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.
Regression?: No
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:
WindowExStyle |= WS_EX_COMPOSITED;
Head over to the existing AnswerHub thread and let us know what's up.