Fix CSS Nesting selector validation

[torresgol10]

Supersedes #30. Continues the work of @jacobcassidy to add CSS Nesting support.

This should fix #9 and #15.

Changes:

Updated css.cson to allow & followed by identifier characters.
Added tests in css-spec.mjs covering nesting scenarios.

[Copilot]

Pull Request Overview

This PR adds CSS Nesting support by fixing the validation of the nesting selector & when followed immediately by other selector parts (e.g., &:hover, &.foo). The grammar now correctly recognizes & as a valid nesting selector and includes it in selector matching patterns throughout the grammar.

Key Changes:

• Added nesting selector & as a valid selector type with scope entity.name.tag.nesting.css
• Updated selector validation patterns to include & in lookahead and lookaround assertions
• Added comprehensive test coverage for CSS nesting scenarios

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
spec/css-spec.mjs	Adds four comprehensive test cases covering the nesting selector & in isolation, with classes, with pseudo-classes, and with combinators
grammars/css.cson	Updates the CSS grammar to recognize & as a valid nesting selector, refactors arithmetic operators, adds function nesting support, and updates selector validation patterns to include & in various contexts

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[Copilot]

Trailing comma after the object in the patterns array. This is invalid CSON syntax and will cause a parsing error.

Suggested change

	},
	}

[torresgol10]

@microsoft-github-policy-service agree

[thernstig]

@torresgol10 does this need to be fixed here?

jacobcassidy/vscode-css-nesting-syntax-highlighting#8

[torresgol10]

Hi @thernstig , thanks for linking that issue! It's an interesting topic.

However, this PR won't fix that problem. The directives mentioned (@variant, @utilities, @apply, etc.) are Tailwind-specific syntax, not standard CSS. This PR focuses solely on native CSS Nesting support as per the W3C specification.

[Athari]

Is this fix to multi-line urls #28 (comment) not integrated into this PR?

				{
					"begin": "\\\\$\\s*?",
					"end": "^(?<!\\G)",
					"name": "constant.character.escape.newline.css"
				},

Considering how long it's taking to get this merged, let's not forget about that tiny thing which completely breaks highlighting in a whole file. 😆

[controversial]

That seems unrelated to this PR

[Athari]

@controversial
It's a trivial single-character fix which the original author of this PR @jacobcassidy intended to include but got distracted with life.

If this single character actually breaks tests or something, I can understand moving it to a separate PR, but if it doesn't, that would seem to be a waste of resources.