fix detection of unused partially global CSS selectors

[Conduitry]

Resolves #4314.

Note that this doesn't actually fix what's described in the OP (which isn't a bug), but rather what I described in my comment - namely, that selectors like :global(.foo) .bar aren't getting correctly marked as unused if nothing matched .bar.

I'm a little concerned that the fix might not be as simple as this PR is making it out to be, but I'm hoping it is.

[dekdevy]

Running into an issue here, i just updated from 3.16 to 3.19.

I have this scss class:

:global(.subnav) {
  display:flex;
  &>input, &>select {
    width: auto;
  }
  &:not(:first-child) {
    margin-top:8px;
  }
}

And this svelte code:

  <div class=subnav>
    <input>
  </div>

---

In 3.16, this is the resulting CSS:

.subnav {
    display: flex
}

.subnav>input,.subnav>select {
    width: auto
}

.subnav:not(:first-child) {
    margin-top: 8px
}

In 3.19, this is the resulting CSS:

.subnav {
    display: flex
}
.subnav:not(:first-child) {
    margin-top: 8px
}

Any ideas?

[Conduitry]

What is the preprocessor turning that into before it gets to the compiler? If you think you can reproduce an issue in the REPL using regular CSS, please open an issue.

[schliflo]

@Conduitry I am having the same issue as @dekdevy mentioned with external scss files that have :global() styles.
I'ts not possible to reproduce via REPL as one cannot create external styles there, but I'd be happy to provide an example repo with minimal setup outlining this issue.
I'm fairly certain this change is responsible for the bug, as everything works as expected in 3.18.0 but breaks starting from 3.18.1 where this change landed.
Let me know if you need an example repo for further investigation.

[Conduitry]

This PR changed the handling of certain styles in <style> blocks within Svelte components. Once preprocessors are run, the styles in these blocks are the only styles that the compiler touches, can see, or knows about. Any change in behavior that is the result of this PR can be seen in a vanilla Svelte component.