In T181816#3926813, @Mvolz wrote:

In T181816#3926809, @Viswaprabha wrote:

The same problem is manifesting and reproducible at ml wiki too. Temporary remedy adapted is to disable both "Automatically enable all new beta features" and "New wikitext mode".
Verified the presence of problem and temporary solution in browsers {(Mozilla FF 57.0.4 (64 bit) | Chrome Version 63.0.3239.132 (Official Build) (64-bit))/ Windows 10}

FYI there's a "turn off syntax highlighting" option so you don't have to turn of wikieditor entirely:

(I am no longer having this issue; not sure what changed but the font is different now.)

The same problem is manifesting and reproducible at ml wiki too. Temporary remedy adapted is to disable both "Automatically enable all new beta features" and "New wikitext mode".
Verified the presence of problem and temporary solution in browsers {(Mozilla FF 57.0.4 (64 bit) | Chrome Version 63.0.3239.132 (Official Build) (64-bit))/ Windows 10}