Problem Description, copied from Original post on mediawiki.org help page:
"It took me a number of edit tests to figure out what these buttons did, because they never seemed to change anything. Specifically, I realised that they expand or collapse blocks of text that aren't part of the edit conflict, but text isn't collapsed unless it reaches a certain length. So if you have an edit conflict that only has one or two (or no) other sentences of text, the buttons appear to do nothing. I'd propose changing the behaviour so that the buttons only show when some text has been collapsed to avoid confusion."
Key findings: If the unchanged text is short, and you press "hide unchanged text" nothing happens, since short text remains unchanged. This is confusing.
Violated Usability principle: Lack of feedback, visibility of system status