Adapt R/A to the window

[TheInvoker]

i use the option to never resize because i don't want the window to resize every time i open a new file,even when they have the same aspect ratio (usually 16:9)
But when i open old 4:3 it would be nice that the window would adapt to either the lenght or the height of the aspect ratio.

Let's say i'm playing a 16:9 1920x1080 video in a window resized to half: 960x540
Then i open a 4:3 640x480 video. Of course i don't want the window to fully resize from 960x540 to 640x480 (that's why i use the never resize option) but i don't even want to see black borders on the sides. So the window should resize at 720x540 (if resizing keeping the height) or at 960x720 (if resizing keeping the lenght)