I would guess that is a reinforcement for the tab, welded to the frame of the sunroof support. Now that is just a guess because I have never even looked at mine unless it was to take it out or put it in. Someone else I am sure will know for sure.
If it is a welded on peace then it is a fairly simple thing to fix if you have a welder or know someone who welds. Just be careful not to melt the roof. Small spot welds in opposite points and wait for them to cool before going to the next spot.
Steve
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Technology is great when it works,
and one big pain in the ass when it doesn't
Detroit iron rules all the rest are just toys.