The length of the I/O fields is defined in the ABAP/4 Dictionary.

"Scrollable" is one of the field attributes available in the Screen Painter which can be set for any field. If you set this attribute, the visible length visLg can be modified. The field is scrollable at runtime, when visLg is < defLg.

The Basis Department is to provide automatic scrolling of a field when entering "=" (matchcode) in an input field, independent of the physically defined length because the matchcode can be longer than the normal field width. For the time being, the physical length of matchcode fields has to be defined wider than the actual visible length.