It's a myth that glass drags downwards over time like a very slow-moving treacle - c/o of people observing that very old windows are thicker at the bottom than they are at the top, and observing that this is not the case with newer windows.
This is a byproduct of the manufacturing processes, whereby old window glass blanks were spun out centrifugally as a giant disc that was necessarily thinner at the edge than at the middle, rather than gravity doing its thing. Obviously it made more sense to have the thicker, heavier, end of the pane at the bottom than at the top. This method was phased out as ones giving better uniformity and less wastage were invented.
Once it's solidified, glass is pretty fixed.