If you put width as 0px, it will take the combined width of the elements inside the "MobileLoginScreen", like the title bar box width, the text "User", the input box for User, and so on. It cannot really be 0 due to the contents inside the Login box.
If that's not the case, you may also check if you are overriding the width of the class MobileLoginScreen in a CSS style sheet under the MIME repository under your Internet Service in se80. For more info on this, check this out: ITS templates – Styles, Themes and Legibility (MIME Repository)
Let me know.