Glass is not fluid enough to perceptively change shape in hundreds of years, if it's fluid at all. There may be some debate on if it would flow over the course of huge, cosmic scales of time, I'm not clear on that, but glass is a solid for all intents and purposes when it comes any earthly scale of time, even geological time.

Stained glass windows have glass panes with one side thicker than the other because of manufacturing defects, not because of fluid dynamics.