Some fonts write underscores below descenders but don't include that in the declared font height, this is technically incorrect and confuses the layout software in Scintilla (the editing widget Geany uses) so insufficient height is allowed for underscores.
To accomodate this you can set some space between lines by editing `Menu->Tools->Configuration Files->filetypes.common` and changing the values on the `line_height` setting, or just use the font that works :smile:.