I have added a few buttons to a screen that displays tabular data but does not use the SAP standard table display. The scrollbar at the right hand side of the screen has now disappeared. Is this a problem in my script or a bug in GuiXT?

This is probably a so-called "steploop" dynpro, a dynpro type that is now rarely used in the SAP system; SAP has changed almost all steploop dynpros into table control dynpros.  In this case, the scrollbar does not depend on the window or screen size, but on the number of elements in the table. GuiXT misinterprets this special scrollbar. Here is a workaround for you to avoid the GuiXT misinterpretation: use the statement NoAreaResize at the beginning of your script.