> Yep, I've confirmed that this is an issue with plain old MAME as well. I tested the > latest windows 64 binary from mamedev.org and have the same issue.
It might be an issue with Windows 10 as relates to the RAWINPUT MAME uses for dealing with inputs by default. Hard to support Win 10 right now since it is only in prerelease. You might try a version of MAME using SDL 2.0 instead to see if the situation improves or not: http://mamedev.emulab.it/haze/2015/02/25/ume-mame-mess-0-159