Recently, Geany suddenly started showing underscores as spaces, i.e., they've become "invisible".
For example, the text is_empty displays the same as is empty. But when copying to the clipboard from Geany, the underscores are as they should. They're only displayed wrong.
Ubuntu 20.04 (Focal Fossa) has the same issue.
The problem is well described on Underscore doesn't appear #1387:
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.
The solution is to either change font or to instruct Geany to make the line height bigger:
Menu Tools → Configuration Files → filetypes.common
[styling]
line_height=0;2;
Note: The problem is fixed in Ubuntu 24.04 (Noble Numbat)