Either windows’ or windows’s is correct, actually. The reason is because of exactly words like “Windows”, if you use the former, it sounds like it’s a possessive of more than one window, but it’s a possessive of a proper noun, Windows. The latter is more correct in this case because of that. (it’s also pronounced that way!)
This is literally on the road map for GIMP, right up top. (Status: no just means it hasn’t been started yet and isn’t planned for 3.2, not that it isn’t planned) https://developer.gimp.org/core/roadmap/