This is a RFC regarding updating to the current version of Scintilla and getting off of the "fork" of the versions 3.x series (LongTerm3 branch).
The longer we wait to update, especially with the splitting off of lexers from Scintilla itself, we're basically just accumulating technical debt. Eventually the kind fellow currently maintaining the LongTerm3 branch will stop.
As far as I know, the main platforms Geany tries to support all have modern enough compilers/C++ standard libraries by now. There may be some older legacy/LTS platforms with users who won't be able to self-update to latest Geany without more effort, but with such limited developer time/interest/contributions for Geany, I don't think it's unreasonable to make some trade-offs like this in order to ensure Geany doesn't bitrot.
I might be willing to have a look at upgrading the code to work with latest Scintilla, but not if people are opposed, hence this RFC.