Documents side-by-side: highlight the active side, for file-saving operation
"Documents side-by-side" is great feature, but I suggest to highlight the active one of both, since both of them can be edited. Otherwise it is easy to mistakely save the wrong file (specially when using [Cmd] + [S], where file name is not shown). The active file name is already displayed in the program window frame, but it would be helpful to implement an additional visual help to see which of both sides (panels) is meant. For example a frame around the panel, in the system-color which becomes blue while active, same as window program frames do it.