Use correct document when creating elements

src/browser/services/Services.ts

@@ -34,6 +34,11 @@ export interface ICoreBrowserService {
    * window.
    */
   readonly window: Window & typeof globalThis;
+  /**
+   * The document of the primary window if working with multiple windows. This
+   * is set by the documentOverride setting.
+   */
+  readonly mainDocument: Document;
   /**
    * Helper for getting the devicePixelRatio of the parent window.
    */