Mint themes are their own
The default Mint theme looks like a fork of Greybird a popular theme designed by some Xfce designers (default Xubuntu theme), which is why I asked again. GTK+ themes are cross-desktop, and since there are so few good ones out there after the giant GTK+3 breakages, it's not uncommon for two different DE users to have the same theme.
its unlikely that themes will make those type of sizing errors
I've gotten all kinds of similar weird errors when using a GTK+ theme which hadn't been updated after GTK+ theme breaks in recent years.
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I think the VTE has been known to cause such errors before, you could try disabling it. You could also try running Geany with `--g-fatal-warnings` in GDB to maybe see where the warning is coming from.