SCORM, closing windows, LMSFinish and synchronous XHR dismissals

Symptom

Upon closing a SCORM course, the LMS doesn't do [something expected] when using a modern browser - typically one based on the Chromium rendering engine (Chrome, Edge, Opera etc.)

SCORM overview

The LMS makes a series of standardised, named functions (via the SCORM API), available in JavaScript with which the content JavaScript can interact. The content does not directly communicate with the LMS except through this SCORM API. The back end of the SCORM API, implemented by the LMS, communicates with the LMS on the course's behalf using http requests.

Problem summary

When the course is closed, the course calls the LMS's SCORM function LMSFinish. In some cases, the LMSFinish function implemented by the LMS uses a synchronous, or blocking, XML-http request to save the data back to the server. Since around March 2020, Chromium-based browsers have prevented synchronous XML http requests in order that whilst a request is being performed, the browser window does not hang. Instead, asynchronous or non-blocking http requests are performed so that the browser window stays performant whilst in use.

Problem use-case

Old method

1. [the user clicks the window close button]
2. Window triggers beforeunload event
   a) LMSFinish starts a blocking request, causing the browser to hang
   b) LMSFinish finishes a blocking request
3. Window triggers unload event
4. Browser closes

New problem

1. [the user clicks the window close button]
2. Window triggers beforeunload event
   a) LMSFinish starts a non-blocking request (which never finishes)
3. Window triggers unload event
4. Browser closes

Bad solution

Call LMSFinish and wait for it to complete before closing the window

We could change the way Adapt works to do an asynchronous (non-blocking) http request first, before any call to window.close but... window.close is only one way of closing the window. We aren't able trap or prevent or stop or delay other methods of closing a window in order to call a non-blocking LMSFinish first.

For example, when a user clicks the window's close button, the JavaScript function window.close is never called, but the window triggers its unload events - which can't be stopped - Adapt calls LMSFinish from a window unload event handler, and then the browser closes. LMSFinish in modern browsers (remember that LMSFinish is implemented by the LMS) is now not allowed to make a blocking http request, so when it does (in the case of this bug), the browser closes before the course's finishing http request is complete.

Interim solution

You could add an additional close button to your course, which calls LMSFinish when clicked (see adapt-close). This won't stop users clicking the window close button and losing their stuff, as this isn't an LMS fix for the LMSFinish function, but it may buy you time until you can fix your LMS.

Best solution

Fix the LMS's LMSFinish function.

References

beforeunload, unload, sendBeacon, fun reading, more fun reading, and more, don't forget this one either fetch#keepalive