I have tried everything I can think of, but no matter what I do, the new widget is nowhere close to the mouse.
Had this problem with a 4.6 project, and did try some other version, but basically, there will be some offset between your cursor mouse and the widget position on screen, and depend on current resolution(1920x1080, 1600x900,…), the offset will be different. You may find the resolution for this problem following these link:
Also, it is best that you test run your game on a different viewport with different resolution before release the final version. UMG maybe one of the most tricky problem if you go deep enough.