I've been busy the last months, but some releases ago I've reported a bug about the behaviour of the mouse pointer when using windowed mode, so I will report again with more detail: And again I remark is not intended behaviour, it's a bug since it works fine in mame.
If you set at your mame.ini the following options:
#
# OSD VIDEO OPTIONS
#
window 1
#
# OSD INPUT OPTIONS
#
mouseprovider win32
When the game is launched with win32, the window captures the pointer, making it no visible and letting you control the gun crosshair (In this example with Operation Wolf). Has been this way in mame and mameui forever, but since some releases ago, MameUI doesn't capture the mouse pointer, making the game unplayable with that devide since you can't move the gun crosshair. It's the same behaviour like setting mouseprovider to dinput.
I provide this little video as a example launching the game in both mame and mameui 0.270 versions.
https://www.mediafire.com/file/4ql6omgyebblw6l/MameAndMameUIMousePointer.mp4/file