fix(compiler): make sure selectors inside container queries are correctly scoped

packages/compiler/src/shadow_css.ts

@@ -571,7 +571,8 @@ export class ShadowCss {
             this._scopeSelector(rule.selector, scopeSelector, hostSelector, this.strictStyling);
       } else if (
           rule.selector.startsWith('@media') || rule.selector.startsWith('@supports') ||
-          rule.selector.startsWith('@document') || rule.selector.startsWith('@layer')) {
+          rule.selector.startsWith('@document') || rule.selector.startsWith('@layer') ||
+          rule.selector.startsWith('@container')) {
         content = this._scopeSelectors(rule.content, scopeSelector, hostSelector);
       } else if (rule.selector.startsWith('@font-face') || rule.selector.startsWith('@page')) {
         content = this._stripScopingSelectors(rule.content);

packages/compiler/test/shadow_css/container_queries_spec.ts

@@ -0,0 +1,55 @@
+/**
+ * @license
+ * Copyright Google LLC All Rights Reserved.
+ *
+ * Use of this source code is governed by an MIT-style license that can be
+ * found in the LICENSE file at https://angular.io/license
+ */
+
+import {ShadowCss} from '@angular/compiler/src/shadow_css';
+
+describe('ShadowCss, container queries', () => {
+  function s(css: string, contentAttr: string, hostAttr: string = '') {
+    const shadowCss = new ShadowCss();
+    return shadowCss.shimCssText(css, contentAttr, hostAttr);
+  }
+
+  it('should scope normal selectors inside an unnamed container rules', () => {
+    const css = `@container max(max-width: 500px) {
+             .item {
+               color: red;
+             }
+           }`;
+    const result = s(css, 'host-a');
+    expect(noIndentation(result)).toEqual(noIndentation(`@container max(max-width: 500px) {
+         .item[host-a] {
+           color: red;
+         }
+       }`));
+  });
+
+  it('should scope normal selectors inside a named container rules', () => {
+    const css = `@container container max(max-width: 500px) {
+             .item {
+               color: red;
+             }
+           }`;
+    const result = s(css, 'host-a');
+    // Note that for the time being we are not scoping the container name itself,
+    // this is something that may or may not be done in the future depending
+    // on how the css specs evolve. Currently as of Chrome 107 it looks like shadowDom
+    // boundaries don't effect container queries (thus the scoping wouldn't be needed)
+    // and this aspect of container queries seems to be still under active discussion:
+    // https://github.com/w3c/csswg-drafts/issues/5984
+    expect(noIndentation(result))
+        .toEqual(noIndentation(`@container container max(max-width: 500px) {
+       .item[host-a] {
+         color: red;
+       }
+     }`));
+  });
+});
+
+function noIndentation(str: string): string {
+  return str.replace(/\n\s+/g, '\n');
+}