I pushed my wip branch to https://github.com/kugel-/geany/tree/scintilla4
Before creating a PR I would like to transition to git-subtree, so that future updates become a simple "git subtree merge" command (merging from a side branch that tracks unmodified scintilla, just importing the upstream tarball to git).
I checked rectangular selections and couldn't notice a difference (selecting and pasting works as before).