On Mon, 8 Jul 2019 11:30:50 -0700 (PDT), John-Del
wrote:

I would have bet almost anything the mosfet is shorted. Put it on an ohmmeter and check for continuity across all the pins.

The transistor responds just as it should to the ohmmeter. I have not
tested it at high voltage or current.

In circuit, I believe the MOSFET (and the capacitor) sees about 640 V
as it turns off. 240 V rectified is about 320 V, and I believe the
transistor seed twice that at turnoff?

Those tiny blue disc 1kv caps are known to both split and blow the device they're across. I stock the 331, 471, 681, 102, and 222 in 2kV

Yep, that's the capacitor type here. I have ordered a kit with 20
different values, 10 of each.