A non written rule that most software follow is: if something changes when you hover over an item (usually meaning "select"), you only need one click to act on

A non written rule that most software follow is: if something changes when you hover over an item (usually meaning "select"), you only need one click to act on it. If nothing changes, you need to double click (because a single click will probably select). There are exceptions of course, but you'll realize that most software works like this, and those who don't are the ones users have more trouble adapting.