The function I tried to use seems to be named "focus selection" in English.
Sorry I missed the information about my computer etc. It is an rather old machine, 2 Years old, i5 CPU, 8 GB RAM, 2*500 MB HDD. Windows 7. So I think I havet to buy a a new computer or go back to GIMP. The simpliest and cheapesdt solution seems to return to GIMP.