This is purely a cosmetic issue, so not very important, but it's goofy.
When I click "Register" a window appears prompting me for the License Code. That prompt is 5750x388 pixels.
I am running dual monitors (a 4k next to a 2k monitor) on Win 10. It appears Neck Diagrams auto sizes the registration dialog box to be the full width of both screens? Not sure but i had to drag it clear over until it spanned both screens to find the Unlock and Cancel buttons.