Geany has no control of ligatures, because it just writes the bytes `f` and `i` and its hidden in the GTK text widget and the font how its displayed.
Note the font says `fi` ligature fixed (see [here](https://fonts.google.com/specimen/Share+Tech+Mono)), are you sure that wherever you "came across" it has the latest?
Possibly changing your locale to `C` might fix it, or maybe GTK3 will fix it when the next windows release happens "soon", but really nothing should be ligating? (is that a word) in a monospace font, ligatures are inherently non-monospace.