What tends to happen is the rubber strip between the glass and metal channel gets hard and the the glass falls out.
I would replace the rubber strip making sure you know where the old one is located on the glass. This was a regular thing with 100 and 200 series cars and we used thinners to lubricate the rubber to refit it and then it would evaporate not leaving a oil residue which would then allow it to slip off again.
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