I can confirm @bencaradocdavies workaround works. This is December, 2021, just installed Debian 11 which ships with Geany 1.37.1 which has the "bug". Using line_height=0;1; + uncommenting [styling] as per workaround, above and works fine for me. Display was quite troublesome without being able to see underscores. Thanks very much for providing it.