diff --git a/src/tests/frontend/helper/multipleUsers.js b/src/tests/frontend/helper/multipleUsers.js
new file mode 100644
index 000000000..08ed50217
--- /dev/null
+++ b/src/tests/frontend/helper/multipleUsers.js
@@ -0,0 +1,147 @@
+'use strict';
+
+helper.multipleUsers = {
+  thisUser: null,
+  otherUser: null,
+
+  // open the same pad on different frames (allows concurrent editions to pad)
+  async init() {
+    // do some cleanup, in case any of the tests failed on the previous run
+    const currentToken = _createTokenForCurrentUser();
+    const otherToken = _createTokenForAnotherUser();
+    _removeExistingTokensFromCookie();
+
+    this.thisUser = {
+      $frame: $('#iframe-container iframe'),
+      token: currentToken,
+      // we'll switch between pads, need to store current values of helper.pad*
+      // to be able to restore those values later
+      padChrome$: helper.padChrome$,
+      padOuter$: helper.padOuter$,
+      padInner$: helper.padInner$,
+    };
+
+    this.otherUser = {
+      token: otherToken,
+    };
+
+    // need to perform as the other user, otherwise we'll get the userdup error message
+    await this.performAsOtherUser(this._createFrameForOtherUser.bind(this));
+  },
+
+  async performAsOtherUser(action) {
+    _startActingLike(this.otherUser);
+    await action();
+    // go back to initial state when we're done
+    _startActingLike(this.thisUser);
+  },
+
+  close() {
+    this.thisUser.$frame.attr('style', ''); // make the default ocopy the full height
+    this.otherUser.$frame.remove();
+  },
+
+  async _loadJQueryCodeForOtherFrame() {
+    const code = await $.get('/static/js/jquery.js');
+
+    // make sure we don't override existing jquery
+    const jQueryCode = `if(typeof $ === "undefined") {\n${code}\n}`;
+    const sendkeysCode = await $.get('/tests/frontend/lib/sendkeys.js');
+    const codesToLoad = [jQueryCode, sendkeysCode];
+
+    this.otherUser.padChrome$ = _getFrameJQuery(codesToLoad, this.otherUser.$frame);
+    this.otherUser.padOuter$ =
+        _getFrameJQuery(codesToLoad, this.otherUser.padChrome$('iframe[name="ace_outer"]'));
+    this.otherUser.padInner$ =
+        _getFrameJQuery(codesToLoad, this.otherUser.padOuter$('iframe[name="ace_inner"]'));
+
+    // update helper vars now that they are available
+    helper.padChrome$ = this.otherUser.padChrome$;
+    helper.padOuter$ = this.otherUser.padOuter$;
+    helper.padInner$ = this.otherUser.padInner$;
+  },
+
+  async _createFrameForOtherUser() {
+    // create the iframe
+    const padUrl = this.thisUser.$frame.attr('src');
+    this.otherUser.$frame = $(`<iframe id="other_pad" src="${padUrl}"></iframe>`);
+
+    // place one iframe (visually) below the other
+    this.thisUser.$frame.attr('style', 'height: 50%');
+    this.otherUser.$frame.attr('style', 'height: 50%; top: 50%');
+    this.otherUser.$frame.insertAfter(this.thisUser.$frame);
+
+    // wait for other pad to load
+    await new Promise((resolve) => this.otherUser.$frame.one('load', resolve));
+
+    const $editorLoadingMessage = this.otherUser.$frame.contents().find('#editorloadingbox');
+    const $errorMessageModal = this.thisUser.$frame.contents().find('#connectivity .userdup');
+
+    await helper.waitForPromise(() => {
+      const finishedLoadingOtherFrame = !$editorLoadingMessage.is(':visible');
+      // make sure we don't get the userdup by mistake
+      const didNotDetectUserDup = !$errorMessageModal.is(':visible');
+
+      return finishedLoadingOtherFrame && didNotDetectUserDup;
+    }, 50000);
+
+    // need to get values for this.otherUser.pad* vars
+    await this._loadJQueryCodeForOtherFrame();
+  },
+};
+
+// adapted form helper.js on Etherpad code
+const _getFrameJQuery = (codesToLoad, $iframe) => {
+  const win = $iframe[0].contentWindow;
+  const doc = win.document;
+
+  for (let i = 0; i < codesToLoad.length; i++) {
+    win.eval(codesToLoad[i]);
+  }
+
+  win.$.window = win;
+  win.$.document = doc;
+
+  return win.$;
+};
+
+const _getDocumentWithCookie = () => (
+  helper.padChrome$
+    ? helper.padChrome$.document
+    : helper.multipleUsers.thisUser.$frame.get(0).contentDocument
+);
+
+const _setTokenOnCookie = (token) => {
+  _getDocumentWithCookie().cookie = `token=${token};secure`;
+};
+
+const _getTokenFromCookie = () => {
+  const fullCookie = _getDocumentWithCookie().cookie;
+  return fullCookie.replace(/.*token=([^;]*).*/, '$1').trim();
+};
+
+const _createTokenForCurrentUser = () => (
+  _getTokenFromCookie().replace(/-other_user.*/g, '')
+);
+
+const _createTokenForAnotherUser = () => {
+  const currentToken = _createTokenForCurrentUser();
+  return `${currentToken}-other_user${helper.randomString(4)}`;
+};
+
+const _startActingLike = (user) => {
+  // update helper references, so other methods will act as if the main frame
+  // was the one we're using from now on
+  helper.padChrome$ = user.padChrome$;
+  helper.padOuter$ = user.padOuter$;
+  helper.padInner$ = user.padInner$;
+
+  _setTokenOnCookie(user.token);
+};
+
+const _removeExistingTokensFromCookie = () => {
+  // Expire cookie, to make sure it is removed by the browser.
+  // See https://developer.mozilla.org/en-US/docs/Web/API/Document/cookie#Example_4_Reset_the_previous_cookie
+  _getDocumentWithCookie().cookie = 'token=foo;expires=Thu, 01 Jan 1970 00:00:00 GMT; path=/p';
+  _getDocumentWithCookie().cookie = 'token=foo;expires=Thu, 01 Jan 1970 00:00:00 GMT; path=/';
+};
diff --git a/src/tests/frontend/index.html b/src/tests/frontend/index.html
index 3d806f626..e8a8012fb 100644
--- a/src/tests/frontend/index.html
+++ b/src/tests/frontend/index.html
@@ -20,6 +20,7 @@
     <script src="helper.js"></script>
     <script src="helper/methods.js"></script>
     <script src="helper/ui.js"></script>
+    <script src="helper/multipleUsers.js"></script>
 
     <script src="specs_list.js"></script>
     <script src="runner.js"></script>
diff --git a/src/tests/frontend/specs/collab_client.js b/src/tests/frontend/specs/collab_client.js
new file mode 100644
index 000000000..307e37908
--- /dev/null
+++ b/src/tests/frontend/specs/collab_client.js
@@ -0,0 +1,102 @@
+'use strict';
+
+describe('Messages in the COLLABROOM', function () {
+  const user1Text = 'text created by user 1';
+  const user2Text = 'text created by user 2';
+
+  const triggerEvent = (eventName) => {
+    const event = new helper.padInner$.Event(eventName);
+    helper.padInner$('#innerdocbody').trigger(event);
+  };
+
+  const replaceLineText = async (lineNumber, newText) => {
+    const inner$ = helper.padInner$;
+
+    // get the line element
+    const $line = inner$('div').eq(lineNumber);
+
+    // simulate key presses to delete content
+    $line.sendkeys('{selectall}'); // select all
+    $line.sendkeys('{del}'); // clear the first line
+    $line.sendkeys(newText); // insert the string
+
+    await helper.waitForPromise(() => inner$('div').eq(lineNumber).text() === newText);
+  };
+
+  before(async function () {
+    this.timeout(10000);
+    await helper.aNewPad();
+    await helper.multipleUsers.init();
+  });
+
+  it('bug #4978 regression test', async function () {
+    // The bug was triggered by receiving a change from another user while simultaneously composing
+    // a character and waiting for an acknowledgement of a previously sent change.
+
+    // User 1 starts sending a change to the server.
+    let sendStarted;
+    const finishSend = (() => {
+      const socketJsonObj = helper.padChrome$.window.pad.socket.json;
+      const sendBackup = socketJsonObj.send;
+      let startSend;
+      sendStarted = new Promise((resolve) => { startSend = resolve; });
+      let finishSend;
+      const sendP = new Promise((resolve) => { finishSend = resolve; });
+      socketJsonObj.send = (...args) => {
+        startSend();
+        sendP.then(() => {
+          socketJsonObj.send = sendBackup;
+          socketJsonObj.send(...args);
+        });
+      };
+      return finishSend;
+    })();
+    await replaceLineText(0, user1Text);
+    await sendStarted;
+
+    // User 1 starts a character composition.
+    triggerEvent('compositionstart');
+
+    // User 1 receives a change from user 2. (User 1 will not incorporate the change until the
+    // composition is completed.)
+    const user2ChangeArrivedAtUser1 = new Promise((resolve) => {
+      const cc = helper.padChrome$.window.pad.collabClient;
+      const origHM = cc.handleMessageFromServer;
+      cc.handleMessageFromServer = (evt) => {
+        if (evt.type === 'COLLABROOM' && evt.data.type === 'NEW_CHANGES') {
+          cc.handleMessageFromServer = origHM;
+          resolve();
+        }
+        return origHM.call(cc, evt);
+      };
+    });
+    await helper.multipleUsers.performAsOtherUser(async () => await replaceLineText(1, user2Text));
+    await user2ChangeArrivedAtUser1;
+
+    // User 1 finishes sending the change to the server. User 2 should see the changes right away.
+    finishSend();
+    await helper.multipleUsers.performAsOtherUser(async () => await helper.waitForPromise(
+        () => helper.padInner$('div').eq(0).text() === user1Text));
+
+    // User 1 finishes the character composition. User 2's change should then become visible.
+    triggerEvent('compositionend');
+    await helper.waitForPromise(() => helper.padInner$('div').eq(1).text() === user2Text);
+
+    // Users 1 and 2 make some more changes.
+    await helper.multipleUsers.performAsOtherUser(async () => await replaceLineText(3, user2Text));
+    await replaceLineText(2, user1Text);
+
+    // All changes should appear in both views.
+    const assertContent = async () => await helper.waitForPromise(() => {
+      const expectedLines = [
+        user1Text,
+        user2Text,
+        user1Text,
+        user2Text,
+      ];
+      return expectedLines.every((txt, i) => helper.padInner$('div').eq(i).text() === txt);
+    });
+    await assertContent();
+    await helper.multipleUsers.performAsOtherUser(assertContent);
+  });
+});