In the address field, if you put in an address, then a comma, and then another address, and then press <tab> for autocomplete... if there is only a single match for the autocomplete, it replaces what you typed with that match.  But if you click outside that field without first pressing <enter> or <tab>, it will wipe away the entire list of addresses except that which you typed last.  The correct expected behavior would be to wipe away just what you typed last, leaving the rest of the line intact.

Steps to reproduce:
1) Compose
2) Click in to address field
3) Type in "john, harry@nowhere.com, "
4) Then type in a name that will autocomplete to only one thing on YOUR system, let's say that was "sally" which completes to "Sally Bright <sally@nowhere.com>"
5) So now you have something like "john, Sally Bright <sally@nowhere.com>"
6) "Abort" that field by now just clicking on the body of the message.

Expected: 
"john, harry@nowhere.com, sally" would remain

Actual:
Only "sally" remains because "john, harry@nowhere.com, " was wiped from the To: field