Move composer contents to threads composer when starting a thread?

### Your use case

#### What would you like to do?
1. compose a message
2. realize you want it to go to a thread
3. click "start thread" on a message
4. composer contents are moved to thread composer

#### Why would you like to do it?
to match
1. compose message
2. click reply
3. send message as reply

### Have you considered any alternatives?

unintended side effect is that the following does not work anymore:
1. start composing a message
2. pause to write a different message as reply (start thread) to another message
3. click "start thread" on a message
4. compose and send threaded message
5. complete original message

### Additional context

idk, but seems worth thinking about.