The text should disappear from the typing box as soon as the send button is pressed.
The text in the typing box is disappeared only when the message is sent to the recipient. It takes a little bit of extra time to disappear from the box rather than disappearing right away as the send button is pressed.
Sometimes you will see it even taking a second or two to disappear, sometimes it will disappear just after the message is sent. But it will always disappear late than expected.
