I didn't understand the issue completely yet but anyway maybe it's worth to try SciTE (https://archlinux.org/packages/extra/x86_64/scite/) to check whether it has the same problem. If so, it is very likely related to Scintilla. If not, it may or may not be related to Scintilla.
@poluyan when you are bisecting, maybe check with a commit *before* 19336d22946b77bbcb5131544ea0e6317cdcf6b5, this was the latest Scintilla update.