SLEEP_MODE_IDLE return without entering sleep

Everything works fine with Standby and Power-Down, the device goes to sleep forever without waking up
```
void loop() {
SLPCTRL.CTRLA |= SLPCTRL_SMODE_PDOWN_gc; //SLEEP_MODE_PWR_DOWN
sleep_enable();
sleep_cpu();
sleep_disable();

Serial.println("ERROR"); //If you see this there is an error
Serial.flush();
}
```

But I would like to have `millis()` running since I need to keep track of the time
I change to `SLEEP_MODE_IDLE` (`SLPCTRL.CTRLA |= SLPCTRL_SMODE_STDBY_gc;`)to have the main clock running
However, after that, the device just exits sleep mode immediately. And output "ERROR" to the terminal

Two question that need help:
1) Is there any config I need to do for the MCU to go to IDEL mode and enter sleep?
2) While in STANDBY and PWR_DOWN, is there anyway to keep `millis()` active?

Thanks all!

Start with the reported loop and the SLPCTRL sleep-mode settings, then trace how sleep_cpu() behaves for SLEEP_MODE_IDLE, STANDBY, and PWR_DOWN. Check whether the selected mode permits the clock source used by millis(); done means explaining the immediate return and determining whether millis() can remain active in the requested modes.

Written by the indexing model from the issue text.