try this:
* add a first recipient that does not take the whole first line (could be invalid)
* type 'i' until the new recipient wrap to the next line
* remove the last 'i' so that it goes back to the previous line
* press enter
=> the recipient goes to the second line

This is exactly what this comment means: we take care to have the same width, whether the recipient is "assimilated" or "edited". This way, when the user presses enter, the recipient always stays where it is.

Hope this is clear