Run an exact millis() timer...

Since the timer used for millis() (TIMERB3 on Uno WiFi2 and Every) is not shared with any functionality, couldn't it could be set up in "periodic" mode (instead of free-running with overflow interrupts), thereby producing an interrupt every one millisecond exactly, and removing the need for all the fancy logic in the ISR?

Start by locating the TIMERB3 setup and the millis() timer ISR in the ArduinoCore-megaavr sources. Compare the current free-running overflow approach with periodic one-millisecond interrupts, then verify that millis() remains accurate and existing timer behavior is preserved.

Written by the indexing model from the issue text.