Paper thin. The vent blows right on that edge. Probably dried it out. I only use ONR to clean the inside. A harsh chemical (or rubbing alcohol) could probably cause it. I never noticed it till the other day while sitting in the passenger seat for the first time in a while. No idea how long its been like that. At least easy to just schedule the mobile service from the app on the phone and got an appointment in my driveway for a week later.