I'd like a feature that when you enable it only let's use resize the Smplayer window to always match the proportions of the video.
I am not getting distorted aspect ratio when I'm resizing now if you though that. I do get to keep the aspect ratio of the video. However when you resize you get black borders on the sides unless you manually set the window size to match the video perfectly.
If you enable the setting I'm proposing, if you resize the window horizontally for example, it would automatically also resize vertically to fit the video. In other words, it will always resize in two dimensions, regardless of which side/corner you are extending.