Shifting tabstrips: I have problems with the field coordinates after introducing the command

pos S[TAXI_TABSTRIP_OVERVIEW] (9,0)

Do I still need to use the old screen coordinates in the area below the the tabstrip, and if so, why?

Yes, unlike with group boxes, shifting the tabstrip shifts all elements below the tabstrip but retains the old coordinates in the shifted area. This is not consistent, but cannot be changed in GuiXT without breaking existing scripts. We plan to offer an additional script parameter that will rectify this.