You might be able to escape the newlines with `\n` or maybe using the ASCII code like `\x0A` or such, but I've never tried it.
If it doesn't work and you're motivated to fix it, you could probably easily improve this by changing [this line](https://github.com/geany/geany/blob/1.33.0/src/tagmanager/tm_source_file.c#L...) and/or the one two below to check against only _unescaped_ newlines, instead of all of them.