Sad sunroof part. Where did it come from?

Suddenly, My sunroof won’t close all the way because it impinges on the wind deflector. So now I need to pull the deflector down with my finger and carefully close it. I finally decided to actually look at it and I found this sad little piece of rubber/felt laying next to the track on the front left. It must have something to do with lowering the air deflector. Even if I know where it goes I fear putting it back because it WILL crumble. What to do?