fix: Autoscroll on page when using jump_to_id

This PR adds a URL hash check to useEffect. Previously the anchor tags
that use jump_to_id would remain at the top of the page. As a result,
users would have to manually scroll to the target location or just read
the full page. Now when the page has a URL hash, it will send the hash
to the listener in the iframe. Using the message listener, it receives
an object with offset in the event.data and the page will scroll to the
location provided by offset. This change will impact the Learner in the
New Experience view.

src/courseware/course/sequence/Unit.jsx

@@ -70,6 +70,15 @@ function useLoadBearingHook(id) {
   }, [id]);
 }
 
+export function sendUrlHashToFrame(frame) {
+  const { hash } = window.location;
+  if (hash) {
+    // The url hash will be sent to LMS-served iframe in order to find the location of the
+    // hash within the iframe.
+    frame.contentWindow.postMessage({ hashName: hash }, `${getConfig().LMS_BASE_URL}`);
+  }
+}
+
 function Unit({
   courseId,
   format,
@@ -98,13 +107,13 @@ function Unit({
   } = course;
 
   const dispatch = useDispatch();
-
   // Do not remove this hook.  See function description.
   useLoadBearingHook(id);
 
   // We use this ref so that we can hold a reference to the currently active event listener.
   const messageEventListenerRef = useRef(null);
   useEffect(() => {
+    sendUrlHashToFrame(document.getElementById('unit-iframe'));
     function receiveMessage(event) {
       const { type, payload } = event.data;
       if (type === 'plugin.resize') {
@@ -120,7 +129,7 @@ function Unit({
         setModalOptions(payload);
       } else if (event.data.offset) {
         // We listen for this message from LMS to know when the page needs to
-        // be scroll to another location on the page.
+        // be scrolled to another location on the page.
         window.scrollTo(0, event.data.offset);
       }
     }

src/courseware/course/sequence/Unit.test.jsx

@@ -3,7 +3,7 @@ import { Factory } from 'rosie';
 import {
   initializeTestStore, loadUnit, messageEvent, render, screen, waitFor,
 } from '../../../setupTest';
-import Unit from './Unit';
+import Unit, { sendUrlHashToFrame } from './Unit';
 
 describe('Unit', () => {
   let mockData;
@@ -38,7 +38,6 @@ describe('Unit', () => {
 
   it('renders correctly', () => {
     render(<Unit {...mockData} />);
-
     expect(screen.getByText('Loading learning sequence...')).toBeInTheDocument();
     const renderedUnit = screen.getByTitle(unit.display_name);
     expect(renderedUnit).toHaveAttribute('height', String(0));
@@ -49,7 +48,6 @@ describe('Unit', () => {
 
   it('renders proper message for gated content', () => {
     render(<Unit {...mockData} id={unitThatContainsGatedContent.id} />);
-
     expect(screen.getByText('Loading learning sequence...')).toBeInTheDocument();
     expect(screen.getByText('Loading locked content messaging...')).toBeInTheDocument();
   });
@@ -75,7 +73,6 @@ describe('Unit', () => {
   it('handles receiving MessageEvent', async () => {
     render(<Unit {...mockData} />);
     loadUnit();
-
     // Loading message is gone now.
     await waitFor(() => expect(screen.queryByText('Loading learning sequence...')).not.toBeInTheDocument());
     // Iframe's height is set via message.
@@ -125,4 +122,37 @@ describe('Unit', () => {
       { timeout: 100 },
     )).rejects.toThrowError(/Expected the element to have attribute/);
   });
+
+  it('scrolls to correct place onLoad', () => {
+    document.body.innerHTML = "<iframe id='unit-iframe' />";
+
+    const mockHashCheck = jest.fn(frameVar => sendUrlHashToFrame(frameVar));
+    const frame = document.getElementById('unit-iframe');
+    const originalWindow = { ...window };
+    const windowSpy = jest.spyOn(global, 'window', 'get');
+    windowSpy.mockImplementation(() => ({
+      ...originalWindow,
+      location: {
+        ...originalWindow.location,
+        hash: '#test',
+      },
+    }));
+    const messageSpy = jest.spyOn(frame.contentWindow, 'postMessage');
+    messageSpy.mockImplementation(() => ({ hashName: originalWindow.location.hash }));
+    mockHashCheck(frame);
+
+    expect(mockHashCheck).toHaveBeenCalled();
+    expect(messageSpy).toHaveBeenCalled();
+
+    windowSpy.mockRestore();
+  });
+
+  it('calls useEffect and checkForHash', () => {
+    const mockHashCheck = jest.fn(() => sendUrlHashToFrame());
+    const effectSpy = jest.spyOn(React, 'useEffect');
+    effectSpy.mockImplementation(() => mockHashCheck());
+    render(<Unit {...mockData} />);
+    expect(React.useEffect).toHaveBeenCalled();
+    expect(mockHashCheck).toHaveBeenCalled();
+  });
 });