Multiple attempts to open a thread can discard a comment in progress

[trichoplax]

To reproduce:

1. Use a very slow connection (or simulate this with your browser's developer tools).
2. Click on a thread title below a post.
3. During the long wait for the server response, click on the same thread title that hasn't expanded into a thread yet.
4. Click "Reply to this thread".
5. Start typing a comment but do not click "Add reply".
6. When the second server response arrives, see that the typed comment and the text box it was in both vanish irretrievably.

When I observed this in production, it was on a very fast connection, so the behaviour was due to the server being slow. At such times anyone can be affected by this, without needing a slow connection.

Note that clicking twice is the simple example to allow reproducing this easily. In practice a user may click more than twice, resulting in partially typed comments disappearing more than once in the seconds that follow.

[cellio]

High priority because of the data loss. Possible components of a solution (which should be considered in the broader context of the parent issue):

• busy indicator
• disable the action while in process (prevent the extra click)
• save data temporarily (if we don't disable the action)
• ???