disable_aspect_correction
The option documentation is properly formatted yet
Example: 1 Since: 1.3.8
With some themes with overlays and/or specific scaling options (or explicitly configured viewport), you may want to set the following option to disable the keep_aspect option (it will have no effect and aspect ratio correction is always off).
Source: https://github.com/FrodeSolheim/fs-uae/blob/master/docs/options/disable_aspect_correction