Ignore @tailwind utilities inside @reference

[thecrypticace]

You can use @reference "tailwindcss" or @reference "../path/to/your/css/file.css" to reference your theme for use in @apply, theme(…), etc…

Unfortunatley, because the imported file still contains @tailwind utilities it would trigger a re-scan of the filesystem — even though the use of @reference ensures that no CSS can actually be output by the import itself.

This PR does two things:

• Adds some explicit feature detection tests for what features we pick up in a stylesheet based on the CSS written and how things are imported
• Explicitly ignores @tailwind utilities inside of @reference so it isn't a trigger for file scanning

Because of how Vite itself handles dependencies editing files on disk will still trigger a rebuild of any file using @reference. This is because Vite rebuilds files when any of its transitive dependencies change.

For example, given this Vue file:

<style>
@reference "./styles.css";
</style>
<template> <!-- ... --> </template>

And this stylesheet:

@import "tailwindcss";

The dependency chain looks like this: file.vue -> styles.css -> {all the sources in your project}

Vite sees that a file (e.g. index.html) has changed, thus styles.css needs change, which means file.vue needs to be compiled again as well. Now in reality we depend on the on disk version of styles.css not the compiled version but Vite itself doesn't know that (or have a way to indicate this afaik).

Coming up with a solution to that problem will have to be a separate PR — but there is a workaround:

1. Inline the imports from @import "tailwindcss";

Replace this in your main stylesheet:

@import "tailwindcss";

with this (this is basically what node_modules/tailwindcss/index.css is):

@layer theme, base, components, utilities;

@import 'tailwindcss/theme' layer(theme);
@import 'tailwindcss/preflight' layer(base);
@import 'tailwindcss/utilities' layer(utilities);

/* the rest of your styles imports, styles, etc… */

2. Split your stylesheet into "main" and "theme" parts

Your "theme" is comprised of the @import 'tailwindcss/theme' layer(theme); import, any custom @theme blocks, any @config directives, and any @plugin directives. Move all of these into their own file.

For example, replace this with two files:

@layer theme, base, components, utilities;
@import 'tailwindcss/theme' layer(theme);
@import 'tailwindcss/preflight' layer(base);
@import 'tailwindcss/utilities' layer(utilities);

@theme {
  --color-primary: #c0ffee;
}

@plugin "./my-plugin.js";

/* the rest of your styles imports, styles, etc… */

with a theme file:

@import 'tailwindcss/theme' layer(theme);

/* all your `@theme` stuff goes in this file */
@theme {
  --color-primary: #c0ffee;
}

/* additionally any @config or @plugin does too */
@plugin "./my-plugin.js";

and your main CSS file:

@layer theme, base, components, utilities;
@import './my-theme.css'; /* I replaced this import */
@import 'tailwindcss/preflight' layer(base);
@import 'tailwindcss/utilities' layer(utilities);

/* the rest of your styles imports, styles, etc… */

3. Import only your "theme" file in your Vue components / CSS modules / etc…

<style>
@reference "./my-theme.css";
</style>
<template> <!-- ... --> </template>

Fixes #17693