Do you actually have a mame.ini with the proper screenX setting in the directory structure of Mameui? Confirm that's the only mame.ini. At one point mameui's created version of mame.ini had a different inipath entry than one created with baseline which could be impacting things.
It should still run on 7 without issue.
Created the mame.ini if you need to via baseline 'mame.exe -cc'.
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Re: MAMEUI will not output fullscreen to desired monitor.
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