[core] Fix CSSStyleSheet is not defined error in Node build

.changeset/fifty-monkeys-shop.md

@@ -0,0 +1,6 @@
+---
+'lit-element': patch
+'lit': patch
+---
+
+Fix `CSSStyleSheet is not defined` error that would occur when importing a Lit component in Node when both static `styles` and the `@property` decorator were used.

packages/lit-element/src/test/node-imports.ts

@@ -21,13 +21,26 @@ import 'lit-element/decorators/query-assigned-elements.js';
 import 'lit-element/decorators/query-assigned-nodes.js';
 import 'lit-element/decorators/query-async.js';
 
-import {LitElement, html} from 'lit-element';
-import {customElement} from 'lit-element/decorators.js';
+import {LitElement, html, css} from 'lit-element';
+import {customElement, property} from 'lit-element/decorators.js';
 
 @customElement('my-element')
 export class MyElement extends LitElement {
+  // Include both static styles and a @property decorator in the test. The
+  // @property decorator trigggers class initialization, and if there are also
+  // static styles, it will trigger a test for adopted stylesheets, which could
+  // explode if not handled with care in the node build.
+  static override styles = css`
+    p {
+      color: purple;
+    }
+  `;
+
+  @property()
+  name = 'World';
+
   override render() {
-    return html`Hello World`;
+    return html`<p>Hello ${this.name}</p>`;
   }
 }
 

packages/reactive-element/src/css-tag.ts

@@ -11,10 +11,16 @@ const global = NODE_MODE ? globalThis : window;
  * Whether the current browser supports `adoptedStyleSheets`.
  */
 export const supportsAdoptingStyleSheets =
-  global.ShadowRoot &&
-  (global.ShadyCSS === undefined || global.ShadyCSS.nativeShadow) &&
-  'adoptedStyleSheets' in Document.prototype &&
-  'replace' in CSSStyleSheet.prototype;
+  // In Node mode, it doesn't really matter whether we believe adopted style
+  // sheets are supported or not, because styles are handled separately as part
+  // of SSR anyway; however if we set this to true then we avoid `instanceof
+  // CSSStyleSheet` checks during initialization, meaning we don't need to shim
+  // that global.
+  NODE_MODE ||
+  (global.ShadowRoot &&
+    (global.ShadyCSS === undefined || global.ShadyCSS.nativeShadow) &&
+    'adoptedStyleSheets' in Document.prototype &&
+    'replace' in CSSStyleSheet.prototype);
 
 /**
  * A CSSResult or native CSSStyleSheet.

packages/reactive-element/src/test/node-imports.ts

@@ -22,11 +22,24 @@ import '@lit/reactive-element/decorators/query-assigned-elements.js';
 import '@lit/reactive-element/decorators/query-assigned-nodes.js';
 import '@lit/reactive-element/decorators/query-async.js';
 
-import {customElement} from '@lit/reactive-element/decorators.js';
-import {ReactiveElement} from '@lit/reactive-element';
+import {customElement, property} from '@lit/reactive-element/decorators.js';
+import {ReactiveElement, css} from '@lit/reactive-element';
 
 @customElement('my-element')
-export class MyElement extends ReactiveElement {}
+export class MyElement extends ReactiveElement {
+  // Include both static styles and a @property decorator in the test. The
+  // @property decorator trigggers class initialization, and if there are also
+  // static styles, it will trigger a test for adopted stylesheets, which could
+  // explode if not handled with care in the node build.
+  static override styles = css`
+    p {
+      color: purple;
+    }
+  `;
+
+  @property()
+  name = 'World';
+}
 
 export class MyOtherElement extends ReactiveElement {}
 customElements.define('my-other-element', MyOtherElement);