micro:bit sometimes jumps into Bluetooth mode without triple press or A+B+RESET

[martinwork]

While testing #331, I dragged a new build of MICROBIT.hex (MICROBIT.hex.zip) to the MICROBIT drive.

The first attempt was an error 521. The second try appeared to succeed, but micro:bit jumped straight into Bluetooth/pairing mode.

[martinwork]

I just saw a spontaneous jump to Bluetooth mode, seemingly triggered by a reset from MakeCode when switching browser tabs, while testing microsoft/pxt-microbit#4808 (comment),

I can't repeat this right now. Perhaps I opened another tab just as the WebUSB Download from the first tab finished?

[martinwork]

The problems I have had may have been a result of MakeCode live resetting micro:bit twice. MakeCode beta only resets once

Every reset counts towards a triple press. I added a line to clear RESETREAS, so I could see that it is usually 0x04 (soft reset) for reset press or download, but often 0x0C (soft reset+CPU lock-up) for a successful download after an error 521, in which case resetClickCount contains a large number.

Triple press does not work from long-press-reset-sleep. RESETREAS is 0x40004 and resetClickCount contains a large number.

Worth noting in this context:
https://infocenter.nordicsemi.com/topic/errata_nRF52833_Rev1/ERR/nRF52833/Rev1/latest/anomaly_833_136.html