BTW, a little bit of "gedankenexperiment" would show that, if glass really did flow, the panes would not end up so uniformly thin at the top, and so uniformly t

BTW, a little bit of "gedankenexperiment" would show that, if glass really did flow, the panes would not end up so uniformly thin at the top, and so uniformly thick at the bottom, with a smooth transition in between. Rather, they would sag and warp, eventually becoming non-flat if there was any small deviation from verticality. They would remain thicker for longer around the edges (assuming the pane is embedded solidly in a frame) and any support elements that hold it firmly. The top edge would sag more quickly in the middle. Heck, prevailing winds, if any, would slowly push the pane inwards, like a soap bubble. The neat shallow wedge shape simply cannot be obtained via regular viscous flow in a near-vertical pane. --- Also, if mere gravity could change the shape that much, any load-bearing glass element supporting a heavy item would sag visibly in a much shorter time. I'm thinking of the glass table supporting my very large, very heavy plasma TV.