Currently Geany uses version 5.1.5.
The most important changes are:
Version 5.2.0 adds multithreaded layout to significantly improve performance for very wide lines.
Version 5.2.1 fixes leaks on GTK. Version 5.2.2 on GTK, scroll horizontally with Shift + Scroll Wheel. Version 5.2.3 adds 64-bit safe APIs and fixes scrollbar on GTK with Xorg. Version 5.2.4 fixes failures on GTK with multi-threaded layout. Version 5.3.0 adds change history.
"change history" seems the most interesting.