fix(compiler): handle :host-context with comma-separated child selector (#59276)

Both `:host` and `:host-context` work by looking for a specific character sequence that is terminated by `,` or `{` and replacing selectors inside of it with scoped versions. This is implemented as a regex which isn't aware of things like nested selectors. Normally this is fine for `:host`, because each `:host` produces one scoped selector which doesn't affect any child selectors, however it breaks down with `:host-context` which replaces each instance with two selectors. For example, if we have a selector in the form of `:host-context(.foo) a:not(.a, .b)`, the compiler ends up determining that `.a,` is the end selector and produces `.foo[a-host] a[contenta]:not(.a, .foo [a-host] a[contenta]:not(.a, .b) {}`.

These changes resolve the issue by splitting the CSS alogn top-level commas, processing the `:host-context` in them individually, and stiching the CSS back together.

PR Close #59276

packages/compiler/src/shadow_css.ts

@@ -541,6 +541,41 @@ export class ShadowCss {
    * .foo<scopeName> .bar { ... }
    */
   private _convertColonHostContext(cssText: string): string {
+    const length = cssText.length;
+    let parens = 0;
+    let prev = 0;
+    let result = '';
+
+    // Splits up the selectors on their top-level commas, processes the :host-context in them
+    // individually and stitches them back together. This ensures that individual selectors don't
+    // affect each other.
+    for (let i = 0; i < length; i++) {
+      const char = cssText[i];
+
+      // If we hit a comma and there are no open parentheses, take the current chunk and process it.
+      if (char === ',' && parens === 0) {
+        result += this._convertColonHostContextInSelectorPart(cssText.slice(prev, i)) + ',';
+        prev = i + 1;
+        continue;
+      }
+
+      // We've hit the end. Take everything since the last comma.
+      if (i === length - 1) {
+        result += this._convertColonHostContextInSelectorPart(cssText.slice(prev));
+        break;
+      }
+
+      if (char === '(') {
+        parens++;
+      } else if (char === ')') {
+        parens--;
+      }
+    }
+
+    return result;
+  }
+
+  private _convertColonHostContextInSelectorPart(cssText: string): string {
     return cssText.replace(_cssColonHostContextReGlobal, (selectorText, pseudoPrefix) => {
       // We have captured a selector that contains a `:host-context` rule.
 
@@ -1010,13 +1045,16 @@ const _cssContentUnscopedRuleRe =
 const _polyfillHost = '-shadowcsshost';
 // note: :host-context pre-processed to -shadowcsshostcontext.
 const _polyfillHostContext = '-shadowcsscontext';
-const _parenSuffix = '(?:\\((' + '(?:\\([^)(]*\\)|[^)(]*)+?' + ')\\))?([^,{]*)';
-const _cssColonHostRe = new RegExp(_polyfillHost + _parenSuffix, 'gim');
+const _parenSuffix = '(?:\\((' + '(?:\\([^)(]*\\)|[^)(]*)+?' + ')\\))';
+const _cssColonHostRe = new RegExp(_polyfillHost + _parenSuffix + '?([^,{]*)', 'gim');
+// note: :host-context patterns are terminated with `{`, as opposed to :host which
+// is both `{` and `,` because :host-context handles top-level commas differently.
+const _hostContextPattern = _polyfillHostContext + _parenSuffix + '?([^{]*)';
 const _cssColonHostContextReGlobal = new RegExp(
-  _cssScopedPseudoFunctionPrefix + '(' + _polyfillHostContext + _parenSuffix + ')',
+  `${_cssScopedPseudoFunctionPrefix}(${_hostContextPattern})`,
   'gim',
 );
-const _cssColonHostContextRe = new RegExp(_polyfillHostContext + _parenSuffix, 'im');
+const _cssColonHostContextRe = new RegExp(_hostContextPattern, 'im');
 const _polyfillHostNoCombinator = _polyfillHost + '-no-combinator';
 const _polyfillHostNoCombinatorOutsidePseudoFunction = new RegExp(
   `${_polyfillHostNoCombinator}(?![^(]*\\))`,

packages/compiler/test/shadow_css/host_and_host_context_spec.ts

@@ -257,6 +257,23 @@ describe('ShadowCss, :host and :host-context', () => {
           '{}',
       );
     });
+
+    it('should handle :host-context with comma-separated child selector', () => {
+      expect(shim(':host-context(.foo) a:not(.a, .b) {}', 'contenta', 'a-host')).toEqualCss(
+        '.foo[a-host] a[contenta]:not(.a, .b), .foo [a-host] a[contenta]:not(.a, .b) {}',
+      );
+      expect(
+        shim(
+          ':host-context(.foo) a:not([a], .b), .bar, :host-context(.baz) a:not([c], .d) {}',
+          'contenta',
+          'a-host',
+        ),
+      ).toEqualCss(
+        '.foo[a-host] a[contenta]:not([a], .b), .foo [a-host] a[contenta]:not([a], .b), ' +
+          '.bar[contenta], .baz[a-host] a[contenta]:not([c], .d), ' +
+          '.baz [a-host] a[contenta]:not([c], .d) {}',
+      );
+    });
   });
 
   describe(':host-context and :host combination selector', () => {