We had an appointment with the developer again today and took the opportunity to get information about it. The good news is, there are these extendable FSGs for our windows as well. The bad news is, on one hand, the price (which might be bearable due to the positive benefits), and on the other hand, it was advised against buying because the whole thing is very tricky and once it gets stuck, it apparently quickly slips out of the guide rail and then is broken. So either handle it like a raw egg, look for a sensible alternative, or don’t open the window in the summer...