diff --git a/test/unit/clitests.json b/test/unit/clitests.json
index 9d7352d5a..a64126390 100644
--- a/test/unit/clitests.json
+++ b/test/unit/clitests.json
@@ -23,6 +23,7 @@
     "document_spec.js",
     "encodings_spec.js",
     "evaluator_spec.js",
+    "event_utils_spec.js",
     "function_spec.js",
     "message_handler_spec.js",
     "metadata_spec.js",
diff --git a/test/unit/event_utils_spec.js b/test/unit/event_utils_spec.js
new file mode 100644
index 000000000..50c7e8b5b
--- /dev/null
+++ b/test/unit/event_utils_spec.js
@@ -0,0 +1,286 @@
+/* Copyright 2017 Mozilla Foundation
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+import {
+  EventBus,
+  waitOnEventOrTimeout,
+  WaitOnType,
+} from "../../web/event_utils.js";
+import { isNodeJS } from "../../src/shared/is_node.js";
+
+describe("event_utils", function () {
+  describe("EventBus", function () {
+    it("dispatch event", function () {
+      const eventBus = new EventBus();
+      let count = 0;
+      eventBus.on("test", function (evt) {
+        expect(evt).toEqual(undefined);
+        count++;
+      });
+      eventBus.dispatch("test");
+      expect(count).toEqual(1);
+    });
+    it("dispatch event with arguments", function () {
+      const eventBus = new EventBus();
+      let count = 0;
+      eventBus.on("test", function (evt) {
+        expect(evt).toEqual({ abc: 123 });
+        count++;
+      });
+      eventBus.dispatch("test", {
+        abc: 123,
+      });
+      expect(count).toEqual(1);
+    });
+    it("dispatch different event", function () {
+      const eventBus = new EventBus();
+      let count = 0;
+      eventBus.on("test", function () {
+        count++;
+      });
+      eventBus.dispatch("nottest");
+      expect(count).toEqual(0);
+    });
+    it("dispatch event multiple times", function () {
+      const eventBus = new EventBus();
+      let count = 0;
+      eventBus.dispatch("test");
+      eventBus.on("test", function () {
+        count++;
+      });
+      eventBus.dispatch("test");
+      eventBus.dispatch("test");
+      expect(count).toEqual(2);
+    });
+    it("dispatch event to multiple handlers", function () {
+      const eventBus = new EventBus();
+      let count = 0;
+      eventBus.on("test", function () {
+        count++;
+      });
+      eventBus.on("test", function () {
+        count++;
+      });
+      eventBus.dispatch("test");
+      expect(count).toEqual(2);
+    });
+    it("dispatch to detached", function () {
+      const eventBus = new EventBus();
+      let count = 0;
+      const listener = function () {
+        count++;
+      };
+      eventBus.on("test", listener);
+      eventBus.dispatch("test");
+      eventBus.off("test", listener);
+      eventBus.dispatch("test");
+      expect(count).toEqual(1);
+    });
+    it("dispatch to wrong detached", function () {
+      const eventBus = new EventBus();
+      let count = 0;
+      eventBus.on("test", function () {
+        count++;
+      });
+      eventBus.dispatch("test");
+      eventBus.off("test", function () {
+        count++;
+      });
+      eventBus.dispatch("test");
+      expect(count).toEqual(2);
+    });
+    it("dispatch to detached during handling", function () {
+      const eventBus = new EventBus();
+      let count = 0;
+      const listener1 = function () {
+        eventBus.off("test", listener2);
+        count++;
+      };
+      const listener2 = function () {
+        eventBus.off("test", listener1);
+        count++;
+      };
+      eventBus.on("test", listener1);
+      eventBus.on("test", listener2);
+      eventBus.dispatch("test");
+      eventBus.dispatch("test");
+      expect(count).toEqual(2);
+    });
+
+    it("dispatch event to handlers with/without 'once' option", function () {
+      const eventBus = new EventBus();
+      let multipleCount = 0,
+        onceCount = 0;
+
+      eventBus.on("test", function () {
+        multipleCount++;
+      });
+      eventBus.on(
+        "test",
+        function () {
+          onceCount++;
+        },
+        { once: true }
+      );
+
+      eventBus.dispatch("test");
+      eventBus.dispatch("test");
+      eventBus.dispatch("test");
+
+      expect(multipleCount).toEqual(3);
+      expect(onceCount).toEqual(1);
+    });
+
+    it("should not re-dispatch to DOM", async function () {
+      if (isNodeJS) {
+        pending("Document is not supported in Node.js.");
+      }
+      const eventBus = new EventBus();
+      let count = 0;
+      eventBus.on("test", function (evt) {
+        expect(evt).toEqual(undefined);
+        count++;
+      });
+      function domEventListener() {
+        // Shouldn't get here.
+        expect(false).toEqual(true);
+      }
+      document.addEventListener("test", domEventListener);
+
+      eventBus.dispatch("test");
+
+      await Promise.resolve();
+      expect(count).toEqual(1);
+
+      document.removeEventListener("test", domEventListener);
+    });
+  });
+
+  describe("waitOnEventOrTimeout", function () {
+    let eventBus;
+
+    beforeAll(function () {
+      eventBus = new EventBus();
+    });
+
+    afterAll(function () {
+      eventBus = null;
+    });
+
+    it("should reject invalid parameters", async function () {
+      const invalidTarget = waitOnEventOrTimeout({
+        target: "window",
+        name: "DOMContentLoaded",
+      }).then(
+        function () {
+          // Shouldn't get here.
+          expect(false).toEqual(true);
+        },
+        function (reason) {
+          expect(reason instanceof Error).toEqual(true);
+        }
+      );
+
+      const invalidName = waitOnEventOrTimeout({
+        target: eventBus,
+        name: "",
+      }).then(
+        function () {
+          // Shouldn't get here.
+          expect(false).toEqual(true);
+        },
+        function (reason) {
+          expect(reason instanceof Error).toEqual(true);
+        }
+      );
+
+      const invalidDelay = waitOnEventOrTimeout({
+        target: eventBus,
+        name: "pagerendered",
+        delay: -1000,
+      }).then(
+        function () {
+          // Shouldn't get here.
+          expect(false).toEqual(true);
+        },
+        function (reason) {
+          expect(reason instanceof Error).toEqual(true);
+        }
+      );
+
+      await Promise.all([invalidTarget, invalidName, invalidDelay]);
+    });
+
+    it("should resolve on event, using the DOM", async function () {
+      if (isNodeJS) {
+        pending("Document is not supported in Node.js.");
+      }
+      const button = document.createElement("button");
+
+      const buttonClicked = waitOnEventOrTimeout({
+        target: button,
+        name: "click",
+        delay: 10000,
+      });
+      // Immediately dispatch the expected event.
+      button.click();
+
+      const type = await buttonClicked;
+      expect(type).toEqual(WaitOnType.EVENT);
+    });
+
+    it("should resolve on timeout, using the DOM", async function () {
+      if (isNodeJS) {
+        pending("Document is not supported in Node.js.");
+      }
+      const button = document.createElement("button");
+
+      const buttonClicked = waitOnEventOrTimeout({
+        target: button,
+        name: "click",
+        delay: 10,
+      });
+      // Do *not* dispatch the event, and wait for the timeout.
+
+      const type = await buttonClicked;
+      expect(type).toEqual(WaitOnType.TIMEOUT);
+    });
+
+    it("should resolve on event, using the EventBus", async function () {
+      const pageRendered = waitOnEventOrTimeout({
+        target: eventBus,
+        name: "pagerendered",
+        delay: 10000,
+      });
+      // Immediately dispatch the expected event.
+      eventBus.dispatch("pagerendered");
+
+      const type = await pageRendered;
+      expect(type).toEqual(WaitOnType.EVENT);
+    });
+
+    it("should resolve on timeout, using the EventBus", async function () {
+      const pageRendered = waitOnEventOrTimeout({
+        target: eventBus,
+        name: "pagerendered",
+        delay: 10,
+      });
+      // Do *not* dispatch the event, and wait for the timeout.
+
+      const type = await pageRendered;
+      expect(type).toEqual(WaitOnType.TIMEOUT);
+    });
+  });
+});
diff --git a/test/unit/jasmine-boot.js b/test/unit/jasmine-boot.js
index f73287ec2..7c3e5c6d6 100644
--- a/test/unit/jasmine-boot.js
+++ b/test/unit/jasmine-boot.js
@@ -68,6 +68,7 @@ async function initializePDFJS(callback) {
       "pdfjs-test/unit/document_spec.js",
       "pdfjs-test/unit/encodings_spec.js",
       "pdfjs-test/unit/evaluator_spec.js",
+      "pdfjs-test/unit/event_utils_spec.js",
       "pdfjs-test/unit/function_spec.js",
       "pdfjs-test/unit/fetch_stream_spec.js",
       "pdfjs-test/unit/message_handler_spec.js",
diff --git a/test/unit/pdf_find_controller_spec.js b/test/unit/pdf_find_controller_spec.js
index 73bb10cd9..8c6453aff 100644
--- a/test/unit/pdf_find_controller_spec.js
+++ b/test/unit/pdf_find_controller_spec.js
@@ -14,7 +14,7 @@
  */
 
 import { buildGetDocumentParams } from "./test_utils.js";
-import { EventBus } from "../../web/ui_utils.js";
+import { EventBus } from "../../web/event_utils.js";
 import { getDocument } from "../../src/display/api.js";
 import { PDFFindController } from "../../web/pdf_find_controller.js";
 import { SimpleLinkService } from "../../web/pdf_link_service.js";
diff --git a/test/unit/ui_utils_spec.js b/test/unit/ui_utils_spec.js
index 4b7bf147d..d262f1079 100644
--- a/test/unit/ui_utils_spec.js
+++ b/test/unit/ui_utils_spec.js
@@ -16,16 +16,12 @@
 import {
   backtrackBeforeAllVisibleElements,
   binarySearchFirstItem,
-  EventBus,
   getPageSizeInches,
   getVisibleElements,
   isPortraitOrientation,
   isValidRotation,
   parseQueryString,
-  waitOnEventOrTimeout,
-  WaitOnType,
 } from "../../web/ui_utils.js";
-import { isNodeJS } from "../../src/shared/is_node.js";
 
 describe("ui_utils", function () {
   describe("binary search", function () {
@@ -56,153 +52,6 @@ describe("ui_utils", function () {
     });
   });
 
-  describe("EventBus", function () {
-    it("dispatch event", function () {
-      const eventBus = new EventBus();
-      let count = 0;
-      eventBus.on("test", function (evt) {
-        expect(evt).toEqual(undefined);
-        count++;
-      });
-      eventBus.dispatch("test");
-      expect(count).toEqual(1);
-    });
-    it("dispatch event with arguments", function () {
-      const eventBus = new EventBus();
-      let count = 0;
-      eventBus.on("test", function (evt) {
-        expect(evt).toEqual({ abc: 123 });
-        count++;
-      });
-      eventBus.dispatch("test", {
-        abc: 123,
-      });
-      expect(count).toEqual(1);
-    });
-    it("dispatch different event", function () {
-      const eventBus = new EventBus();
-      let count = 0;
-      eventBus.on("test", function () {
-        count++;
-      });
-      eventBus.dispatch("nottest");
-      expect(count).toEqual(0);
-    });
-    it("dispatch event multiple times", function () {
-      const eventBus = new EventBus();
-      let count = 0;
-      eventBus.dispatch("test");
-      eventBus.on("test", function () {
-        count++;
-      });
-      eventBus.dispatch("test");
-      eventBus.dispatch("test");
-      expect(count).toEqual(2);
-    });
-    it("dispatch event to multiple handlers", function () {
-      const eventBus = new EventBus();
-      let count = 0;
-      eventBus.on("test", function () {
-        count++;
-      });
-      eventBus.on("test", function () {
-        count++;
-      });
-      eventBus.dispatch("test");
-      expect(count).toEqual(2);
-    });
-    it("dispatch to detached", function () {
-      const eventBus = new EventBus();
-      let count = 0;
-      const listener = function () {
-        count++;
-      };
-      eventBus.on("test", listener);
-      eventBus.dispatch("test");
-      eventBus.off("test", listener);
-      eventBus.dispatch("test");
-      expect(count).toEqual(1);
-    });
-    it("dispatch to wrong detached", function () {
-      const eventBus = new EventBus();
-      let count = 0;
-      eventBus.on("test", function () {
-        count++;
-      });
-      eventBus.dispatch("test");
-      eventBus.off("test", function () {
-        count++;
-      });
-      eventBus.dispatch("test");
-      expect(count).toEqual(2);
-    });
-    it("dispatch to detached during handling", function () {
-      const eventBus = new EventBus();
-      let count = 0;
-      const listener1 = function () {
-        eventBus.off("test", listener2);
-        count++;
-      };
-      const listener2 = function () {
-        eventBus.off("test", listener1);
-        count++;
-      };
-      eventBus.on("test", listener1);
-      eventBus.on("test", listener2);
-      eventBus.dispatch("test");
-      eventBus.dispatch("test");
-      expect(count).toEqual(2);
-    });
-
-    it("dispatch event to handlers with/without 'once' option", function () {
-      const eventBus = new EventBus();
-      let multipleCount = 0,
-        onceCount = 0;
-
-      eventBus.on("test", function () {
-        multipleCount++;
-      });
-      eventBus.on(
-        "test",
-        function () {
-          onceCount++;
-        },
-        { once: true }
-      );
-
-      eventBus.dispatch("test");
-      eventBus.dispatch("test");
-      eventBus.dispatch("test");
-
-      expect(multipleCount).toEqual(3);
-      expect(onceCount).toEqual(1);
-    });
-
-    it("should not re-dispatch to DOM", async function () {
-      if (isNodeJS) {
-        pending("Document is not supported in Node.js.");
-      }
-      const eventBus = new EventBus();
-      let count = 0;
-      eventBus.on("test", function (evt) {
-        expect(evt).toEqual(undefined);
-        count++;
-      });
-      function domEventListener() {
-        // Shouldn't get here.
-        expect(false).toEqual(true);
-      }
-      document.addEventListener("test", domEventListener);
-
-      eventBus.dispatch("test");
-
-      await Promise.resolve();
-      expect(count).toEqual(1);
-
-      document.removeEventListener("test", domEventListener);
-    });
-  });
-
   describe("isValidRotation", function () {
     it("should reject non-integer angles", function () {
       expect(isValidRotation()).toEqual(false);
@@ -290,122 +139,6 @@ describe("ui_utils", function () {
     });
   });
 
-  describe("waitOnEventOrTimeout", function () {
-    let eventBus;
-
-    beforeAll(function () {
-      eventBus = new EventBus();
-    });
-
-    afterAll(function () {
-      eventBus = null;
-    });
-
-    it("should reject invalid parameters", async function () {
-      const invalidTarget = waitOnEventOrTimeout({
-        target: "window",
-        name: "DOMContentLoaded",
-      }).then(
-        function () {
-          // Shouldn't get here.
-          expect(false).toEqual(true);
-        },
-        function (reason) {
-          expect(reason instanceof Error).toEqual(true);
-        }
-      );
-
-      const invalidName = waitOnEventOrTimeout({
-        target: eventBus,
-        name: "",
-      }).then(
-        function () {
-          // Shouldn't get here.
-          expect(false).toEqual(true);
-        },
-        function (reason) {
-          expect(reason instanceof Error).toEqual(true);
-        }
-      );
-
-      const invalidDelay = waitOnEventOrTimeout({
-        target: eventBus,
-        name: "pagerendered",
-        delay: -1000,
-      }).then(
-        function () {
-          // Shouldn't get here.
-          expect(false).toEqual(true);
-        },
-        function (reason) {
-          expect(reason instanceof Error).toEqual(true);
-        }
-      );
-
-      await Promise.all([invalidTarget, invalidName, invalidDelay]);
-    });
-
-    it("should resolve on event, using the DOM", async function () {
-      if (isNodeJS) {
-        pending("Document is not supported in Node.js.");
-      }
-      const button = document.createElement("button");
-
-      const buttonClicked = waitOnEventOrTimeout({
-        target: button,
-        name: "click",
-        delay: 10000,
-      });
-      // Immediately dispatch the expected event.
-      button.click();
-
-      const type = await buttonClicked;
-      expect(type).toEqual(WaitOnType.EVENT);
-    });
-
-    it("should resolve on timeout, using the DOM", async function () {
-      if (isNodeJS) {
-        pending("Document is not supported in Node.js.");
-      }
-      const button = document.createElement("button");
-
-      const buttonClicked = waitOnEventOrTimeout({
-        target: button,
-        name: "click",
-        delay: 10,
-      });
-      // Do *not* dispatch the event, and wait for the timeout.
-
-      const type = await buttonClicked;
-      expect(type).toEqual(WaitOnType.TIMEOUT);
-    });
-
-    it("should resolve on event, using the EventBus", async function () {
-      const pageRendered = waitOnEventOrTimeout({
-        target: eventBus,
-        name: "pagerendered",
-        delay: 10000,
-      });
-      // Immediately dispatch the expected event.
-      eventBus.dispatch("pagerendered");
-
-      const type = await pageRendered;
-      expect(type).toEqual(WaitOnType.EVENT);
-    });
-
-    it("should resolve on timeout, using the EventBus", async function () {
-      const pageRendered = waitOnEventOrTimeout({
-        target: eventBus,
-        name: "pagerendered",
-        delay: 10,
-      });
-      // Do *not* dispatch the event, and wait for the timeout.
-
-      const type = await pageRendered;
-      expect(type).toEqual(WaitOnType.TIMEOUT);
-    });
-  });
-
   describe("getPageSizeInches", function () {
     it("gets page size (in inches)", function () {
       const page = {
diff --git a/web/app.js b/web/app.js
index 451151b86..0bd425669 100644
--- a/web/app.js
+++ b/web/app.js
@@ -18,10 +18,8 @@ import {
   animationStarted,
   apiPageLayoutToViewerModes,
   apiPageModeToSidebarView,
-  AutomationEventBus,
   AutoPrintRegExp,
   DEFAULT_SCALE_VALUE,
-  EventBus,
   getActiveOrFocusedElement,
   isValidRotation,
   isValidScrollMode,
@@ -37,6 +35,7 @@ import {
   TextLayerMode,
 } from "./ui_utils.js";
 import { AppOptions, compatibilityParams, OptionKind } from "./app_options.js";
+import { AutomationEventBus, EventBus } from "./event_utils.js";
 import {
   build,
   createPromiseCapability,
diff --git a/web/event_utils.js b/web/event_utils.js
new file mode 100644
index 000000000..077699598
--- /dev/null
+++ b/web/event_utils.js
@@ -0,0 +1,196 @@
+/* Copyright 2012 Mozilla Foundation
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+const WaitOnType = {
+  EVENT: "event",
+  TIMEOUT: "timeout",
+};
+
+/**
+ * @typedef {Object} WaitOnEventOrTimeoutParameters
+ * @property {Object} target - The event target, can for example be:
+ *   `window`, `document`, a DOM element, or an {EventBus} instance.
+ * @property {string} name - The name of the event.
+ * @property {number} delay - The delay, in milliseconds, after which the
+ *   timeout occurs (if the event wasn't already dispatched).
+ */
+
+/**
+ * Allows waiting for an event or a timeout, whichever occurs first.
+ * Can be used to ensure that an action always occurs, even when an event
+ * arrives late or not at all.
+ *
+ * @param {WaitOnEventOrTimeoutParameters}
+ * @returns {Promise} A promise that is resolved with a {WaitOnType} value.
+ */
+function waitOnEventOrTimeout({ target, name, delay = 0 }) {
+  return new Promise(function (resolve, reject) {
+    if (
+      typeof target !== "object" ||
+      !(name && typeof name === "string") ||
+      !(Number.isInteger(delay) && delay >= 0)
+    ) {
+      throw new Error("waitOnEventOrTimeout - invalid parameters.");
+    }
+
+    function handler(type) {
+      if (target instanceof EventBus) {
+        target._off(name, eventHandler);
+      } else {
+        target.removeEventListener(name, eventHandler);
+      }
+
+      if (timeout) {
+        clearTimeout(timeout);
+      }
+      resolve(type);
+    }
+
+    const eventHandler = handler.bind(null, WaitOnType.EVENT);
+    if (target instanceof EventBus) {
+      target._on(name, eventHandler);
+    } else {
+      target.addEventListener(name, eventHandler);
+    }
+
+    const timeoutHandler = handler.bind(null, WaitOnType.TIMEOUT);
+    const timeout = setTimeout(timeoutHandler, delay);
+  });
+}
+
+/**
+ * Simple event bus for an application. Listeners are attached using the `on`
+ * and `off` methods. To raise an event, the `dispatch` method shall be used.
+ */
+class EventBus {
+  constructor() {
+    this._listeners = Object.create(null);
+  }
+
+  /**
+   * @param {string} eventName
+   * @param {function} listener
+   * @param {Object} [options]
+   */
+  on(eventName, listener, options = null) {
+    this._on(eventName, listener, {
+      external: true,
+      once: options?.once,
+    });
+  }
+
+  /**
+   * @param {string} eventName
+   * @param {function} listener
+   * @param {Object} [options]
+   */
+  off(eventName, listener, options = null) {
+    this._off(eventName, listener, {
+      external: true,
+      once: options?.once,
+    });
+  }
+
+  /**
+   * @param {string} eventName
+   * @param {Object} data
+   */
+  dispatch(eventName, data) {
+    const eventListeners = this._listeners[eventName];
+    if (!eventListeners || eventListeners.length === 0) {
+      return;
+    }
+    let externalListeners;
+    // Making copy of the listeners array in case if it will be modified
+    // during dispatch.
+    for (const { listener, external, once } of eventListeners.slice(0)) {
+      if (once) {
+        this._off(eventName, listener);
+      }
+      if (external) {
+        (externalListeners ||= []).push(listener);
+        continue;
+      }
+      listener(data);
+    }
+    // Dispatch any "external" listeners *after* the internal ones, to give the
+    // viewer components time to handle events and update their state first.
+    if (externalListeners) {
+      for (const listener of externalListeners) {
+        listener(data);
+      }
+      externalListeners = null;
+    }
+  }
+
+  /**
+   * @ignore
+   */
+  _on(eventName, listener, options = null) {
+    const eventListeners = (this._listeners[eventName] ||= []);
+    eventListeners.push({
+      listener,
+      external: options?.external === true,
+      once: options?.once === true,
+    });
+  }
+
+  /**
+   * @ignore
+   */
+  _off(eventName, listener, options = null) {
+    const eventListeners = this._listeners[eventName];
+    if (!eventListeners) {
+      return;
+    }
+    for (let i = 0, ii = eventListeners.length; i < ii; i++) {
+      if (eventListeners[i].listener === listener) {
+        eventListeners.splice(i, 1);
+        return;
+      }
+    }
+  }
+}
+
+/**
+ * NOTE: Only used to support various PDF viewer tests in `mozilla-central`.
+ */
+class AutomationEventBus extends EventBus {
+  dispatch(eventName, data) {
+    if (typeof PDFJSDev !== "undefined" && !PDFJSDev.test("MOZCENTRAL")) {
+      throw new Error("Not implemented: AutomationEventBus.dispatch");
+    }
+    super.dispatch(eventName, data);
+
+    const details = Object.create(null);
+    if (data) {
+      for (const key in data) {
+        const value = data[key];
+        if (key === "source") {
+          if (value === window || value === document) {
+            return; // No need to re-dispatch (already) global events.
+          }
+          continue; // Ignore the `source` property.
+        }
+        details[key] = value;
+      }
+    }
+    const event = document.createEvent("CustomEvent");
+    event.initCustomEvent(eventName, true, true, details);
+    document.dispatchEvent(event);
+  }
+}
+
+export { AutomationEventBus, EventBus, waitOnEventOrTimeout, WaitOnType };
diff --git a/web/pdf_history.js b/web/pdf_history.js
index 4a49ee5a7..7fdc1e459 100644
--- a/web/pdf_history.js
+++ b/web/pdf_history.js
@@ -17,8 +17,8 @@ import {
   isValidRotation,
   parseQueryString,
   PresentationModeState,
-  waitOnEventOrTimeout,
 } from "./ui_utils.js";
+import { waitOnEventOrTimeout } from "./event_utils.js";
 
 // Heuristic value used when force-resetting `this._blockHashChange`.
 const HASH_CHANGE_TIMEOUT = 1000; // milliseconds
diff --git a/web/pdf_viewer.component.js b/web/pdf_viewer.component.js
index a424c29e7..63be97d16 100644
--- a/web/pdf_viewer.component.js
+++ b/web/pdf_viewer.component.js
@@ -29,16 +29,17 @@ import {
   DefaultXfaLayerFactory,
   XfaLayerBuilder,
 } from "./xfa_layer_builder.js";
-import { EventBus, ProgressBar } from "./ui_utils.js";
 import { PDFLinkService, SimpleLinkService } from "./pdf_link_service.js";
 import { PDFSinglePageViewer, PDFViewer } from "./pdf_viewer.js";
 import { DownloadManager } from "./download_manager.js";
+import { EventBus } from "./event_utils.js";
 import { GenericL10n } from "./genericl10n.js";
 import { NullL10n } from "./l10n_utils.js";
 import { PDFFindController } from "./pdf_find_controller.js";
 import { PDFHistory } from "./pdf_history.js";
 import { PDFPageView } from "./pdf_page_view.js";
 import { PDFScriptingManager } from "./pdf_scripting_manager.js";
+import { ProgressBar } from "./ui_utils.js";
 
 // eslint-disable-next-line no-unused-vars
 const pdfjsVersion = PDFJSDev.eval("BUNDLE_VERSION");
diff --git a/web/ui_utils.js b/web/ui_utils.js
index 4c9a057cc..c662f84cf 100644
--- a/web/ui_utils.js
+++ b/web/ui_utils.js
@@ -632,63 +632,6 @@ function isPortraitOrientation(size) {
   return size.width <= size.height;
 }
 
-const WaitOnType = {
-  EVENT: "event",
-  TIMEOUT: "timeout",
-};
-
-/**
- * @typedef {Object} WaitOnEventOrTimeoutParameters
- * @property {Object} target - The event target, can for example be:
- *   `window`, `document`, a DOM element, or an {EventBus} instance.
- * @property {string} name - The name of the event.
- * @property {number} delay - The delay, in milliseconds, after which the
- *   timeout occurs (if the event wasn't already dispatched).
- */
-
-/**
- * Allows waiting for an event or a timeout, whichever occurs first.
- * Can be used to ensure that an action always occurs, even when an event
- * arrives late or not at all.
- *
- * @param {WaitOnEventOrTimeoutParameters}
- * @returns {Promise} A promise that is resolved with a {WaitOnType} value.
- */
-function waitOnEventOrTimeout({ target, name, delay = 0 }) {
-  return new Promise(function (resolve, reject) {
-    if (
-      typeof target !== "object" ||
-      !(name && typeof name === "string") ||
-      !(Number.isInteger(delay) && delay >= 0)
-    ) {
-      throw new Error("waitOnEventOrTimeout - invalid parameters.");
-    }
-
-    function handler(type) {
-      if (target instanceof EventBus) {
-        target._off(name, eventHandler);
-      } else {
-        target.removeEventListener(name, eventHandler);
-      }
-
-      if (timeout) {
-        clearTimeout(timeout);
-      }
-      resolve(type);
-    }
-
-    const eventHandler = handler.bind(null, WaitOnType.EVENT);
-    if (target instanceof EventBus) {
-      target._on(name, eventHandler);
-    } else {
-      target.addEventListener(name, eventHandler);
-    }
-
-    const timeoutHandler = handler.bind(null, WaitOnType.TIMEOUT);
-    const timeout = setTimeout(timeoutHandler, delay);
-  });
-}
-
 /**
  * Promise that is resolved when DOM window becomes visible.
  */
@@ -706,129 +649,6 @@ const animationStarted = new Promise(function (resolve) {
   window.requestAnimationFrame(resolve);
 });
 
-/**
- * Simple event bus for an application. Listeners are attached using the `on`
- * and `off` methods. To raise an event, the `dispatch` method shall be used.
- */
-class EventBus {
-  constructor() {
-    this._listeners = Object.create(null);
-  }
-
-  /**
-   * @param {string} eventName
-   * @param {function} listener
-   * @param {Object} [options]
-   */
-  on(eventName, listener, options = null) {
-    this._on(eventName, listener, {
-      external: true,
-      once: options?.once,
-    });
-  }
-
-  /**
-   * @param {string} eventName
-   * @param {function} listener
-   * @param {Object} [options]
-   */
-  off(eventName, listener, options = null) {
-    this._off(eventName, listener, {
-      external: true,
-      once: options?.once,
-    });
-  }
-
-  /**
-   * @param {string} eventName
-   * @param {Object} data
-   */
-  dispatch(eventName, data) {
-    const eventListeners = this._listeners[eventName];
-    if (!eventListeners || eventListeners.length === 0) {
-      return;
-    }
-    let externalListeners;
-    // Making copy of the listeners array in case if it will be modified
-    // during dispatch.
-    for (const { listener, external, once } of eventListeners.slice(0)) {
-      if (once) {
-        this._off(eventName, listener);
-      }
-      if (external) {
-        (externalListeners ||= []).push(listener);
-        continue;
-      }
-      listener(data);
-    }
-    // Dispatch any "external" listeners *after* the internal ones, to give the
-    // viewer components time to handle events and update their state first.
-    if (externalListeners) {
-      for (const listener of externalListeners) {
-        listener(data);
-      }
-      externalListeners = null;
-    }
-  }
-
-  /**
-   * @ignore
-   */
-  _on(eventName, listener, options = null) {
-    const eventListeners = (this._listeners[eventName] ||= []);
-    eventListeners.push({
-      listener,
-      external: options?.external === true,
-      once: options?.once === true,
-    });
-  }
-
-  /**
-   * @ignore
-   */
-  _off(eventName, listener, options = null) {
-    const eventListeners = this._listeners[eventName];
-    if (!eventListeners) {
-      return;
-    }
-    for (let i = 0, ii = eventListeners.length; i < ii; i++) {
-      if (eventListeners[i].listener === listener) {
-        eventListeners.splice(i, 1);
-        return;
-      }
-    }
-  }
-}
-
-/**
- * NOTE: Only used to support various PDF viewer tests in `mozilla-central`.
- */
-class AutomationEventBus extends EventBus {
-  dispatch(eventName, data) {
-    if (typeof PDFJSDev !== "undefined" && !PDFJSDev.test("MOZCENTRAL")) {
-      throw new Error("Not implemented: AutomationEventBus.dispatch");
-    }
-    super.dispatch(eventName, data);
-
-    const details = Object.create(null);
-    if (data) {
-      for (const key in data) {
-        const value = data[key];
-        if (key === "source") {
-          if (value === window || value === document) {
-            return; // No need to re-dispatch (already) global events.
-          }
-          continue; // Ignore the `source` property.
-        }
-        details[key] = value;
-      }
-    }
-    const event = document.createEvent("CustomEvent");
-    event.initCustomEvent(eventName, true, true, details);
-    document.dispatchEvent(event);
-  }
-}
-
 function clamp(v, min, max) {
   return Math.min(Math.max(v, min), max);
 }
@@ -988,14 +808,12 @@ export {
   apiPageLayoutToViewerModes,
   apiPageModeToSidebarView,
   approximateFraction,
-  AutomationEventBus,
   AutoPrintRegExp,
   backtrackBeforeAllVisibleElements, // only exported for testing
   binarySearchFirstItem,
   DEFAULT_SCALE,
   DEFAULT_SCALE_DELTA,
   DEFAULT_SCALE_VALUE,
-  EventBus,
   getActiveOrFocusedElement,
   getOutputScale,
   getPageSizeInches,
@@ -1023,7 +841,5 @@ export {
   TextLayerMode,
   UNKNOWN_SCALE,
   VERTICAL_PADDING,
-  waitOnEventOrTimeout,
-  WaitOnType,
   watchScroll,
 };