Not sure about the resize issue, but from your screenshot it looks like it has a bunch of code all on one line. Geany/Scintilla is notoriously slow with extremely long lines, especially when line wrapping is enabled.
Aside from trying to disable line-wrapping, there's a couple things you can possibly do if you control whatever generated that code. If you are using C++11 or later you can use [raw string literals](https://en.cppreference.com/w/cpp/language/string_literal) or if not use plain C-style string literal concatenation (described on same link). Both would allow to split the generated code onto multiple lines which should help with the Geany performance issue as well as to make the generated code more readable.