@alevesely would you mind opening an issue about this topic for Scintilla directly?
I'm not interested in RTL editing. It is undoubtedly a relevant topic which will better editing for a lot of people who speak those languages. However, I'm not interested in that.
The main point is the security concerns that [Trojan Source](https://www.trojansource.codes/) brought up. Geany fails to handle that code properly, which might happen to alert a user that there's something wrong with that file. Thus, by chance, Geany provides a rickety protection against that kind of attacks. Until scintilla handles RTL, Geany could just fail more elegantly and provide a more solid protection.