This may be caused by MAME automatically cropping the screen in arcade games and certain home systems, rather than emulating the real hardware's full output (I assume it's going to be even more noticeable when outputting to TVs).
I think that a universal solution to this kind of problems could be to add an overscan slider which would allow to display everything (even the black borders which are actually output by systems such as the SNES), for cases like this.
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