Ensure that the Application.start static method uses overridden class

src/core/application.ts

@@ -17,7 +17,7 @@ export class Application implements ErrorHandler {
   debug = false
 
   static start(element?: Element, schema?: Schema): Application {
-    const application = new Application(element, schema)
+    const application = new this(element, schema)
     application.start()
     return application
   }

src/tests/modules/core/extending_application_tests.ts

@@ -0,0 +1,51 @@
+import { Application } from "../../../core/application"
+import { DOMTestCase } from "../../cases/dom_test_case"
+import { ActionDescriptorFilter } from "src/core/action_descriptor"
+
+const mockCallback: { (label: string): void; lastCall: string | null } = (label: string) => {
+  mockCallback.lastCall = label
+}
+mockCallback.lastCall = null
+
+class TestApplicationWithCustomBehavior extends Application {
+  registerActionOption(name: string, filter: ActionDescriptorFilter): void {
+    mockCallback(`registerActionOption:${name}`)
+    super.registerActionOption(name, filter)
+  }
+}
+
+export default class ExtendingApplicationTests extends DOMTestCase {
+  application!: Application
+
+  async runTest(testName: string) {
+    try {
+      // use the documented way to start & reference only the returned application instance
+      this.application = TestApplicationWithCustomBehavior.start(this.fixtureElement)
+      await super.runTest(testName)
+    } finally {
+      this.application.stop()
+    }
+  }
+
+  async setup() {
+    mockCallback.lastCall = null
+  }
+
+  async teardown() {
+    mockCallback.lastCall = null
+  }
+
+  async "test extended class method is supported when using MyApplication.start()"() {
+    this.assert.equal(mockCallback.lastCall, null)
+
+    const mockTrue = () => true
+    this.application.registerActionOption("kbd", mockTrue)
+    this.assert.equal(this.application.actionDescriptorFilters["kbd"], mockTrue)
+    this.assert.equal(mockCallback.lastCall, "registerActionOption:kbd")
+
+    const mockFalse = () => false
+    this.application.registerActionOption("xyz", mockFalse)
+    this.assert.equal(this.application.actionDescriptorFilters["xyz"], mockFalse)
+    this.assert.equal(mockCallback.lastCall, "registerActionOption:xyz")
+  }
+}