over on geany issues i posted an issue: https://github.com/geany/geany/issues/2359
it seems that when a macro is bound to F8 then the standard GTK event still gets fired after the macro is run.
in the case of F8 it is "give focus to the splitter bar in a paned window" and this event causes the focus on the editor window to be lost, which renders the F8 (or any other combination of a "gtk focus" key) unusable as a macro key.
it appears the standard behaviour in geany for keyboard shortcuts is to *not* fire the GTK event if a shortcut is defined. perhaps the macros plugin should follow the same convention?
a workaround is of course possible: don't use macro keys conflicting with the gtk events: https://developer.gnome.org/hig/stable/keyboard-input.html.en i only discovered geany today and i almost immediately decided to migrate across to it so in the meantime i'll be retraining my brain to not use F8 from my old editor :)
fyi: whilst i was trying to understand the issue i was searching around a bit and found this: https://mail.gnome.org/archives/gtk-app-devel-list/2006-April/msg00338.html an old post which may still be of help.