and I'm not sure tempered glass will still flow.
Glass is in fact a semi-fluid solid...but whether it actually flows or not is a huge debate. I work with stained glass, and that debate comes up a lot. See, old stained glass was all hand made and rolled, so the glass wasn't a perfect uniform thickness liek the machine made stuff is today...same goes for old-time clear glass for normal windows.
Then workers would install the thick side in the bottom, so as to not make the window top heavy, causing a possible stress fracture in the future.
I haven't ever seen really old windows, stained or normal panes, to see if it really does flow down with gravity over several decades.
I think someone needs to call Myth Busters