Adding this as a comment, for anyone similarly affected.

Geany was using ~20% CPU, constantly and consistently when idle -- when launched from the XFCE "Whiskers" menu (which is the way I normally start it), but NOT from terminal (e/g/ xfce4-terminal)!

Solution: edit the Whiskers menu launcher for Geany >> uncheck "Use startup notification." Geany CPU usage (Geany idle) now 0%.