auto-revert-mode, due to magit-mode
been using auto-revert-mode in past month. Pretty good. It automatically update buffer when file on disk changed (by some other program or editor)
not sure why it's on. I looked at my init files but it's not there. Must be some packages that turned it on, but not sure what. Maybe magit-mode did it, since i recall upgrading it. A web search confirmed it. Magit did this sometimes in Feb. https://groups.google.com/forum/#!topic/magit/shCpTvyyygI
i did notice that some buffers don't auto revert, but after reading the magit change discussion , it seems magit turned it on only for files under git.
so, for better global experience, better turn it on globally.