Make compose box topic empty when starting from interleaved stream view #4764
Description
At present, when one starts composing a message from an interleaved stream view, the topic defaults to the topic of the latest message in the stream.
This is confusing, and means that the user has to be careful to change it if, e.g., they just want to start a new topic. It's especially confusing that the compose box uses the topic of the most recent message even if you're currently viewing earlier messages in a totally different topic.
When starting from an interleaved stream view, we should change the behavior to make the topic box blank and put the cursor there to start with.
The typeahead will help if you are trying to respond to a topic, or you can respond to a topic from a non-interleaved topic view.