CSS modules scoped and global class name selectors aren't combined when minified

This:

.a {
    color: red;
}

.b {
    color: red;
}

:global(.c) {
    color: red;
}

:global(.d) {
    color: red;
}

minifies to this:

._8Z4fiW_a, ._8Z4fiW_b {
  color: red;
}

.c {
  color: red;
}

.d {
  color: red;
}

But I expected it to minify to this:

._8Z4fiW_a, ._8Z4fiW_b, .c, .d {
  color: red;
}

Demo:

use lightningcss::{
    css_modules,
    stylesheet::{MinifyOptions, ParserOptions, PrinterOptions, StyleSheet},
};

fn main() {
    let mut stylesheet = StyleSheet::parse(
        r#"
.a {
  color: red;
}

.b {
  color: red;
}

:global(.c) {
  color: red;
}

:global(.d) {
  color: red;
}
"#,
        ParserOptions {
            css_modules: Some(css_modules::Config::default()),
            ..ParserOptions::default()
        },
    )
    .unwrap();

    stylesheet.minify(MinifyOptions::default()).unwrap();
    let css = stylesheet.to_css(PrinterOptions::default()).unwrap();

    // The scoped class name selectors get combined, but the global class name
    // ones don't.
    let expected = r"._8Z4fiW_a, ._8Z4fiW_b {
  color: red;
}

.c {
  color: red;
}

.d {
  color: red;
}
";
    assert_eq!(css.code, expected);

    // As a workaround, I can create a separate style sheet and minify that to
    // combine the scoped and global class name selectors into a single rule.
    let mut stylesheet_2 = StyleSheet::parse(&css.code, ParserOptions::default()).unwrap();
    stylesheet_2.minify(MinifyOptions::default()).unwrap();
    let css_2 = stylesheet_2.to_css(PrinterOptions::default()).unwrap();
    let expected_2 = r"._8Z4fiW_a, ._8Z4fiW_b, .c, .d {
  color: red;
}
";
    assert_eq!(css_2.code, expected_2);
}

Start by running the Rust demo with StyleSheet::parse, css_modules::Config, minify, and to_css to reproduce the selector output. Trace how CSS Modules and global selectors are represented during minification. Done means equivalent scoped and global class selectors combine into one rule, matching expected_2, with regression coverage for this case.