Finding information was hard1 and I had to leave the NLP aspects aside to focus on co-occurrence2. That way, I was able to find several questions or documents which make use of "fenêtre glissante".
"Glissant" here would mean "sliding": the idea is that the window is not something fixed, it moves to accompany your movement while you count the occurrences3.
This seems to match what I understand from your description, but I would be more at ease if someone more knowledgeable of co-occurrence than me could confirm these are the same thing.
1 I am not sure I fully understand all the concepts, especially how the matrix would be calculated without a ramped window.
2 I found French accepts only "cooccurrence" while English also accepts "co-occurrence". I will use the later for readability (it helps distinguishing from "concurrence" for fast readers.
3 Once more, having been unable to find a definition for the phrase, I extrapolate from common uses of "fenêtre" and "glissant".