Okay, this is where we're at now:

Around noon today I made sure the pot was full and then opened both valves, suction side and pressure side, but left the pump off. When I checked it around 16:00 the pot was half empty. There's no evidence of a water leak.

I filled it, sealed it, turned the pump on low. What I saw were many air bubbles, a lot of action in the pot, and a gurgling sound coming from the filter that I hadn't heard before.

There is plenty of water in the pool, halfway up the skimmer opening. I decided to backwash. Dropped the water hose in to add water. Removed the weir—it seems to impede water flow when the pump runs on high and that's the backwash setting.

Turned the pump on high. The pot filled itself to its total maximum with just the smallest air bubble under the top of the domed lid. I ran it through two backwash and rinse cycles, then set the pump back to low.

As of now, the pot is still full, the pump is on and there's no gurgling in the filter. I'm wondering whether the weir door gets stuck and doesn't let enough water into the skimmer? I left it off for now.

Such drama!