fix: optimize scroll position observer after video fullscreen exit (openedx#1405)

Author: ihor-romaniuk

src/courseware/course/sequence/Unit/hooks/useIFrameBehavior.js

@@ -50,19 +50,19 @@ const useIFrameBehavior = ({
     if (type === messageTypes.resize) {
       setIframeHeight(payload.height);
 
-      // We observe exit from the video xblock fullscreen mode
-      // and 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) {
           onLoaded();
         }
       }
     } else if (type === messageTypes.videoFullScreen) {
+      // We observe exit from the video xblock fullscreen mode
+      // and scroll to the previously saved scroll position
+      if (!payload.open && windowTopOffset !== null) {
+        window.scrollTo(0, Number(windowTopOffset));
+      }
+
       // We listen for this message from LMS to know when we need to
       // save or reset scroll position on toggle video xblock fullscreen mode
       setWindowTopOffset(payload.open ? window.scrollY : null);

src/courseware/course/sequence/Unit/hooks/useIFrameBehavior.test.js

@@ -154,6 +154,9 @@ describe('useIFrameBehavior hook', () => {
         const resizeMessage = (height = 23) => ({
           data: { type: messageTypes.resize, payload: { height } },
         });
+        const videoFullScreenMessage = (open = false) => ({
+          data: { type: messageTypes.videoFullScreen, payload: { open } },
+        });
         const testSetIFrameHeight = (height = 23) => {
           const { cb } = useEventListener.mock.calls[0][1];
           cb(resizeMessage(height));
@@ -209,7 +212,7 @@ describe('useIFrameBehavior hook', () => {
           state.mockVals({ ...defaultStateVals, windowTopOffset });
           hook = useIFrameBehavior(props);
           const { cb } = useEventListener.mock.calls[0][1];
-          cb(resizeMessage());
+          cb(videoFullScreenMessage());
           expect(window.scrollTo).toHaveBeenCalledWith(0, windowTopOffset);
         });
         it('does not scroll if towverticalp offset is not set', () => {