select[size=1] {

    overflow: visible;

}

The above code is present in theme/boost/scss/moodle/forms.scss file.

It's invalid CSS and SCSS according to online validators, the correct version would have the 1 inside quotes.

I noticed because I was using the scss compiler provided by npm to pre-complie my .scss files to find errors when I saved my files and it threw an error on this line.

I guess it could also be reported upstream to the PHP Sass compiler library?