This could handle identifying that the underlying file has changed at save time, but currently Geany identifies that the file changed a few tens of seconds after it happens. Then it is more convenient for the user to do something about it, rather than doing a large editing session then finding there is a problem.
Oh... I think is not good idea identifies that the file changed a few tens of seconds after it happens. For example I watch log file which is really changed every second. Notification every tens of seconds would be very annoying. Another case two developers edited source file directly. Of course be better use here version control but my suggestion also can help here solve conflicts.
-- Best Regards, Mike Gavrilov.