Catweazle
I disagree that it is a fallacy that glass is a supercooled fluid.

At school in the physics laboratory we had a large measuring cylinder ~18" long that had been lying undisturbed on its side in a cupboard for well over 100 years, it had sagged in the centre by approximately half an inch; it had been maintained like this as an example of the behaviour of supercooled fluids.

Further, I can point out locally a number of panes of glass in windows up to 350 years old that have become noticeably thinner at the top and thicker at the bottom;, I have often seen the same phenomenon in ancient buildings all over the British Isles.
Kpar
I concur with Catweazle, my own education included much of the same information. Glass flows slowly over time.
Username
Catweazle - It comes down to the definition of liquid. A lot of solids will sag over a period of time.
WB1200
If you put glass into a high speed centrifuge will it sag?
Douglas Bennett Rogers
Cathedral panes were made wedge shaped to be picked up with tongs. Telescope mirrors and lenses retain their shape to parts per million over decades. There are glass beads that are billions of years old that are still bead shaped. There may be glasses that are much more flowable than these examples.
Pete Davis
Normal glass is an amorphous solid, just like wax, some plastics, gel, etc. End of story. It's not a fluid, super-cooled or otherwise.
Lamar Havard
So, is it sticky?
Expanded Viewpoint
In a science book I used to have many moons ago, it said that if you took a glass rod or a tube about 4' long and .25" diameter, and supported it at both ends, then hung a weight in the center that was near the breaking point of the glass, after about a month or so, the glass will have taken a curved set to it.
Gravity never shuts off, so with the constant pulling down on all molecules in an object, they should eventually succumb to it and find their lowest level.

Randy
mr.sugg
Sorry Catweazle and Kpar. The reason why old building glass is heavier at the bottom is because of the old method of manufacture prior to sheet glass on molten tin. The glass was spun into a circle - getting thinner at the circumference due to centrifugal force and after cooling was cut into small panes - hence natural distortion and the use of leadlight.
JasonStanton
It's an obvious phenomenon no matter what the nature of it.