Thanks. I'm fairly sure that's not monospaced text though: it looks like a proportional font to me. (My eyesight's not great though, so I could be wrong).
The gtk-widgets css file for the theme also already contains this:
/* work around a bug with GtkSourceView using a monospaced font for these (pluma etc) /
/ see Ubuntu MATE 19.04 Beta is out! */
.context-menu {
font-weight: normal;
font-family: regular;
}
which I'm guessing is the bugfix you're talking about.
So the problem seems to be that pluma / GTK is translating "font-family: regular" as "font-family: some random font rather than the menu font". neh?