There are two global/admin settings for the forum acitivity: forum_shortpost and forum_longpost
The settings are accompanied by the following description (emphasize by me):
- Short post (Default: 300): Any post under this length (in characters not including HTML) is considered short (see below)
- Long post (Default: 600): Any post over this length (in characters not including HTML) is considered long. Posts displayed on the site front page, social format course pages, or user profiles are shortened to a natural break somewhere between the forum_shortpost and forum_longpost values.
I believe that this behaviour does not exist anymore, I could not get it to work on my local 3.7.1 install as well as demo.moodle.net (with 100 and 150 chars):
The "200-char-post" should be shortened – if I understand this setting correctly – but isn't: