fix: save scroll position on exit from video xblock fullscreen mode

src/courseware/course/sequence/Unit.jsx

@@ -93,6 +93,7 @@ const Unit = ({
   const [showError, setShowError] = useState(false);
   const [modalOptions, setModalOptions] = useState({ open: false });
   const [shouldDisplayHonorCode, setShouldDisplayHonorCode] = useState(false);
+  const [windowTopOffset, setWindowTopOffset] = useState(null);
 
   const unit = useModel('units', id);
   const course = useModel('coursewareMeta', courseId);
@@ -120,6 +121,13 @@ const Unit = ({
     } = data;
     if (type === 'plugin.resize') {
       setIframeHeight(payload.height);
+
+      // We observe exit from the video xblock full screen mode
+      // and do page scroll to the previously saved scroll position
+      if (windowTopOffset !== null) {
+        window.scrollTo(0, Number(windowTopOffset));
+      }
+
       if (!hasLoaded && iframeHeight === 0 && payload.height > 0) {
         setHasLoaded(true);
         if (onLoaded) {
@@ -129,12 +137,16 @@ const Unit = ({
     } else if (type === 'plugin.modal') {
       payload.open = true;
       setModalOptions(payload);
+    } else if (type === 'plugin.videoFullScreen') {
+      // We listen for this message from LMS to know when we need to
+      // save or reset scroll position on toggle video xblock full screen mode.
+      setWindowTopOffset(payload.open ? window.scrollY : null);
     } else if (data.offset) {
       // We listen for this message from LMS to know when the page needs to
       // be scrolled to another location on the page.
       window.scrollTo(0, data.offset + document.getElementById('unit-iframe').offsetTop);
     }
-  }, [id, setIframeHeight, hasLoaded, iframeHeight, setHasLoaded, onLoaded]);
+  }, [id, setIframeHeight, hasLoaded, iframeHeight, setHasLoaded, onLoaded, setWindowTopOffset, windowTopOffset]);
   useEventListener('message', receiveMessage);
   useEffect(() => {
     sendUrlHashToFrame(document.getElementById('unit-iframe'));

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

@@ -129,6 +129,21 @@ describe('Unit', () => {
     expect(window.scrollY === testMessageWithOffset.offset);
   });
 
+  it('scrolls page on MessagaeEvent when receiving videoFullScreen state', async () => {
+    // Set message to constain video full screen data.
+    const defaultTopOffset = 800;
+    const testMessageWithOtherHeight = { ...messageEvent, payload: { height: 500 } };
+    const testMessageWithFullscreenState = (isOpen) => ({ type: 'plugin.videoFullScreen', payload: { open: isOpen } });
+    render(<Unit {...mockData} />);
+    Object.defineProperty(window, 'scrollY', { value: defaultTopOffset, writable: true });
+    window.postMessage(testMessageWithFullscreenState(true), '*');
+    window.postMessage(testMessageWithFullscreenState(false), '*');
+    window.postMessage(testMessageWithOtherHeight, '*');
+
+    await expect(waitFor(() => expect(window.scrollTo()).toHaveBeenCalledTimes(1)));
+    expect(window.scrollY === defaultTopOffset);
+  });
+
   it('ignores MessageEvent with unhandled type', async () => {
     // Clone message and set different type.
     const testMessageWithUnhandledType = { ...messageEvent, type: 'wrong type' };