Recipient inserted in To: disappears

Opening the compose window to the maximum size, the recipient inserted disappears, only if there is NO text in the subject field or in the body of the message.

Steps to reproduce:

• open window for composing mail
• enter only recipient in To:
• click the (central) icon to expand the window to its maximum size

Expected behaviour:

• the recipient entered is still present

Actual behaviour:

• the recipient entered has disappeared

If there is text in the subject field or in the body of the message, the problem is not there.
It seems to happen only on Windows systems (with any browser) and not on desktop linux systems.

Thanks to Emiliano Vavassori (Ticket ref: #138089)