Google Colab suddenly started highlighting some characters in my code in yellow (see picture). When clicking over the yellow mark, a popup message appears stating that "The character could be confused with the ASCII character...which is more common in source code". A disable option appears too in the same popup window but it does not seem to work. In addition, highlighting the code by pointing with the mouse switched from light blue to grey (as also shown in the picture).

What caused these changes? How to revert back?

I tried to select "adjust options" and then "disable highlighting of ambiguous characters" without success. I also tried to clear cache and browsing history. I finally tried to switch browsers.