New message window loses focus when previous message window disappears after sending email

I'm running Postbox beta 14 on Windows and Mac, and something has been bugging me about Postbox on Windows for a while now:

1. Start a new message. Attach a file so that it will take a long-ish time to send.
2. Send the message.
3. Start a new message while the previous message is still sending.

EXPECTED: The window of the email sent in step 2 should disappear, and leave the new window opened in step 3 untouched.
ACTUAL: When the window of the email sent in step 2 closes, the focus is stolen from the new message window.

This is annoying because I'm usually in the middle of typing a new message when the focus is stolen, and it's quite a pain having to restore the focus to the message window and pick up where I left off.

Any chance of a fix?