Pads each column to its widest cell and rebuilds the separator row, keeping :--, :-: and --:. Paste a whole document: only tables are touched, and tables inside fenced code are skipped. A table whose body row has more cells than the header is left as written, because widening it would change what GFM renders. Width counts code points, so wide CJK characters and emoji will look off in a monospace font.