External interrupt fires early?

Using Arduino Microcontrollers
15

AHA! That app note was right. Setting the sense control bit (ISCxx) causes the interrupt. Using the code below, I didn't see it earlier, but by adding delay(1) I see it now. Wasn't seeing it before no doubt because the code was picking up EIFR before the four clock cycles it takes to respond to an interrupt.

#include <Streaming.h>    //http://arduiniana.org/libraries/streaming/

#define LED LED_BUILTIN

volatile boolean int1Flag;
boolean ledState;
uint8_t eifr1, eifr2;

void setup(void)
{
    delay(1000);
    Serial.begin(115200);
    pinMode(LED, OUTPUT);

    cli();
    EICRA = _BV(ISC11);          //external interrupt 1 on falling edge
    delay(1);            //   <------- AHA!
    eifr1 = EIFR;
    EIMSK = _BV(INT1);           //enable external interrupt 1
    eifr2 = EIFR;
    EIFR = _BV(INTF1);
    sei();
    
    Serial << "EIFR1=0x" << _HEX(eifr1) << " EIFR2=0x" << _HEX(eifr2) << endl;
}

void loop(void)
{
    if (int1Flag) {
        int1Flag = false;
        digitalWrite(LED, ledState = !ledState);
        Serial << millis() << endl;
    }
}

ISR(INT1_vect)
{
    int1Flag = true;
}

Output:

EIFR1=0x2 EIFR2=0x2
1909
2910
3909
4910