Yup, I just set up a clean RetroPie 4.7.1 card, and the defaults give full 4:3 for Snes.
Not for Megadrive though: for the lr_genesis_plus_gx default configuration the game image is not quite 4:3 - skinny black bars at left and right. The default config probably wants aspect_ratio_index=0 (for 4:3). Or a little overscan?
i wouldn't want to make assumptions. it could be true to the real hardware. you'd have to drill into the core options and/or the code, but since we use 'core provided' as our default, and generally use default core options, the default aspect ratio is at the discretion of the core developer, so it's typically intentional.
@dankcushions well thanks you for assisting me with this one. I eventually got it working by using the USA Rom and, as you suggested, stretching the image to 5:4. it fills the whole screen and looks good. I'll be happy with that solution for now. Happy Holidays.
Playing with the sourcecode, I usurped the 8:7 PAR to be just 8:7 like 4:3. The switch that controls this is a boolean and I haven't the time to change it right now (and then I would need to make a github account to request a pull for the changes).
I don't think it would be 81, but you are correct that it's wrong. Shouldn't be 16 because my TV only displays up to 720p, not 1080p. Although it is a 16:9 TV (not 64:27 like it shows under #81). Also, my TV frequency isn't 30Hz, unless I'm missing something.
you've fixed it now so i guess this is irrelevant but yes, you'd need to use hdmi_group=2 for 81 - it doesn't seem to be supported under 1. but yes, @mitu 's suggestion would get show you the complete list of supported modes. given that it's less than 4k, i would always want to be running in the native panel resolution so you don't get scaling artefacts.
i assume now you've taken out hdmi_safe you can safely take out any hard coded modes and it should hopefully choose 1366x768 by default.