I2C does not work after save (Espruino Pico)

Your understanding of the waveforms is correct.

I used save() : when I'm done debugging, I (again) download the code from the IDE and immediately issue a 'save()'. I assume that puts the Espruino JavaScript engine in a 'virgin' state without too much debug history left behind.

I agree with your comment that there may not be much time gained implementing an I2C event (except for large data chunks such as displays). I find it always a challenge to take into account the lower speed of a small CPU compared to the desktop beasts we use to develop our hardware/firmware :).

A combined 'writeTo - readFrom' might be an easier cycle-saver : an I2C read transaction almost always starts with write-register-address - but that's only something to add to the whish-list...

   i2c.writeTo(i2c_address, register_address);
   var data = i2c.readFrom(i2c_address, count);
// whish :
  var data = i2c.readFromAddress(i2c_address, register_address, count);