OK, so here's my take on the file preview. I like the st3 way of doing it better than the st2 way. With st2, it could get confusing because if you had a file open for editing, and clicked on a file to preview, it looked (at least with some themes) that the file you were previewing was a file opened for editing, and named the same as the file you had been editing. But, I think that it should be more apparent that the file being previewed in st3 is in preview mode, and as has been mentioned, if I navigate away from the tab, it should close automatically. In other words, preview should not be functionally equivalent to opening the file for editing, or there shouldn't be a preview mode at all.