Version 4.5.3967 uses a version of Monaco which puts a box around non-standard invisible tokens like non-breaking spaces, including zero-width ones, so there's no need to underline these. In addition, errors should not be underlined as this was a cludge to highlight invisible characters, but with 19.0's output of error messages in error colour, it makes for a very bad experience:
Version 4.5.3967 uses a version of Monaco which puts a box around non-standard invisible tokens like non-breaking spaces, including zero-width ones, so there's no need to underline these. In addition, errors should not be underlined as this was a cludge to highlight invisible characters, but with 19.0's output of error messages in error colour, it makes for a very bad experience: