That’s a misconception. Glass does not flow over time; it is a rigid, amorphous solid. The uneven thickness seen in old cathedral windows is a result of historical manufacturing. Glass was spun into discs that naturally became thicker at the edges, and builders installed the heavier side at the bottom for stability. Physicists calculate that it would take longer than the age of the universe for room-temperature window glass to visibly deform under gravity. Reference: Zanotto, E. D. (1998). "Do cathedral glasses flow?" American Journal of Physics, https://doi.org/10.1119/1.19026