Thank you, Stiletto. I've only been here for 14 years and never knew that.
Well... MESSUI never _used_ to be Robbbert's responsibility to maintain, it used to be an officially supported project of the MESS development team.
(Unlike MAMEUI, which merely had its own development team - it wasn't supported by MAME.)
But ever since MAME and MESS developers decided (years ago now) to halt the intertwining of their projects with the MAME32/MAMEUI codebase... (due to many reasons but chiefly due to user interface design issues with MESS, and MAMEUI's and MESSUI's main developers leaving to work on other things)... MESSUI was removed from MESS's sourcecode and became, mostly, a dead project. Perhaps you missed the arguments about that on here.
It's only kept alive by Robbbert's efforts, although people like Mamesick (MAMEUIFX) also work on their own MAMEUI-based projects, also keeping it alive. John IV still updates the MAMEUI project, but as there are no actual MAMEUI developers driving development, but hackers applying quick fixes ensuring the whole ball of wax doesn't fall apart...
No offense to Robbbert, but you're honestly much better off using MESS itself from command-line, or learning to use it with a modern frontend such as QMC2. My $0.02.
Still, MESSUI will be around as people like Robbbert continue patching it to the latest MESS code and compiling it.
Anyhow, all that is a bit off-topic for this forum these days.