Assuming that it is okay to "zip up" articles of clothing on Shabbos (e.g. pants), is the act of zipping not considered Tofer at all, or is it just permitted because it lacks permanence?
According to Halachipedia this is disputed by halachic authorities:
Some poskim maintain that is permitted to use a zipper on Shabbat. Others, however, say that it is permitted to use a zipper only if it is meant to be unzipped in a short time.
Shibbolei HaLeket (108; cited in Beit Yosef 340:7) writes that it is permitted to connect two pieces of clothing using loops and hooks and it isn’t considered Tofer. Based on this Shibbolei HaLeket, Yalkut Yosef (Shabbat vol. 5 pg. 140) permits using zippers on Shabbat, since the connection between the two pieces only is an external connection with interlocking pieces similar to loops and hooks. Rabbi Mordechai Willig (Halachipedia Article 5773 #13), Shemirat Shabbat KeHilchata 15:74, Sh”t Betzel Chachma 3:92, 39 Melachos (Rabbi Ribiat, vol 3, pg 818), and The Shabbos Home (Rabbi Simcha Cohen, vol 1, pg 75) agree. Rabbi Sobolofsky (“Tofer 2” min 7-8) said that another reason to be lenient is because its normal usage is to zip and unzip it, similar to closing a door (see Mishna Brurah 340:17).
Rav Soloveitchik (quoted by Rav Hershel Schachter, Halachipedia Article 5773 #13) suggested that since the small zipper teeth interlock so much so that it becomes like one material, it could be an issue of Tofer. He added that using a zipper that one uses all the time is permitted just like a very temporary knot. Sheivet HaLevi 3:51, Rav Yaakov Kamenetsky (Emet LeYacov 317:3), and Rav Yisrael Belsky (cited by [Halachically Speaking 3:14:3) agree but add that one shouldn’t zip a lining into one’s winter coat, as that is meant to be left for a long period of time. Rav Hershel Shechter (quoted by Rabbi Rosner between minutes 37:30 and 38:30) is strict for this opinion and doesn’t zipper his Shabbat Tallit bag because of this. Rabbi Rosner (between minutes 36:30 and 37:30) explained that even those who are strict to consider zippers Tofer would be lenient regarding pants zippers because anything which will be unzipped in 24 hours is considered temporary.