When mpv is used with smplayer, it disables mpv's default ability to draw outside the picture without a way to turn it back on. When I complained about it to mpv developers, they replied something along the lines: "smplayer and similar GUIs that use deprecated technique of intergration instead of shared library tend to force obsolete options that do more harm than good with which we will not going to deal". But smplayer's option to add "block borders", that seem to be internally called as "letterbox" but actually is "expand" filter, fixes that problem. However, it can't be used by default: option to enable in all times it doesn't save and the one that does pertain only to fullscreen which looks ugly on fs/window switching.
So, please, either let entire filter configuration be saved (as in https://sourceforge.net/p/smplayer/feature-requests/583/) or make the persistent option universal.
You also may want to reconsider forcing "no-keepaspect" with mpv. And "pp" filter is obsolete in mpv, if I correctly recall.
Well, there's no option to enable the black borders by default in window mode, but there's an option that enables the black borders in fullscreen: preferences -> general -> video.
Yeah, I just wrote about that:
There is no reason not to have it all the time. Unless there are bugs with ASS subtitle rendering or something.
You're right. I think there's no reason to limit the filter to fullscreen. I'll try to fix it soon.
For now I've made a crude patch for it.
An option to use black borders by default has been added to version 16.11.