fix: parse nested selectors without & (#1144)

* fix: parse nested selectors without &

Parse nested selector rules that omit & so scoping and :global() handling apply correctly.

Fixes withastro/astro#10721

* Add comment

Author: matthewp

.changeset/tame-lemons-nest.md

@@ -0,0 +1,5 @@
+---
+"@astrojs/compiler": patch
+---
+
+Fix CSS nesting so nested selectors without an ampersand are parsed and scoped correctly.

internal/transform/scope-css_test.go

@@ -270,6 +270,11 @@ func TestScopeStyle(t *testing.T) {
 			source: ".header { background-color: white; &.dark { background-color: blue; }}",
 			want:   ".header:where(.astro-xxxxxx){background-color:white;&.dark{background-color:blue}}",
 		},
+		{
+			name:   "nesting without ampersand",
+			source: ".nesting-root{p{color:#123456}:global(h1){color:#abcdef}}",
+			want:   ".nesting-root:where(.astro-xxxxxx){p:where(.astro-xxxxxx){color:#123456}h1{color:#abcdef}}",
+		},
 		{
 			name: "@container",
 			source: `@container (min-width: 200px) and (min-height: 200px) {

lib/esbuild/css_parser/css_parser.go

@@ -308,11 +308,25 @@ func (p *parser) parseListOfDeclarations() (list []css_ast.Rule) {
 			list = append(list, p.parseSelectorRuleFrom(p.index, parseSelectorOpts{allowNesting: true}))
 
 		default:
-			list = append(list, p.parseDeclaration())
+			if p.shouldParseNestedSelector() {
+				list = append(list, p.parseSelectorRuleFrom(p.index, parseSelectorOpts{allowNesting: true}))
+			} else {
+				list = append(list, p.parseDeclaration())
+			}
 		}
 	}
 }
 
+func (p *parser) shouldParseNestedSelector() bool {
+	clone := *p
+	clone.log = logger.Log{AddMsg: func(msg logger.Msg) {}}
+	// Peek ahead to treat declarations that actually start a nested rule as selector rules.
+	if _, ok := clone.parseSelectorList(parseSelectorOpts{allowNesting: true}); !ok {
+		return false
+	}
+	return clone.peek(css_lexer.TOpenBrace)
+}
+
 func mangleRules(rules []css_ast.Rule) []css_ast.Rule {
 	type hashEntry struct {
 		indices []uint32