So I've been trying for the last few days to write a test to reproduce the error I've sometimes observed. However I was unable to get there with vanilla javascript :-/

Therefore I'm posting first here the observation:

https://github.com/HtmlUnit/htmlunit/blob/master/src/main/java/com/gargoylesoftware/htmlunit/html/HtmlPage.java#L2513

if (elementWithFocus_ != null) {
            if (getWebClient().isJavaScriptEnabled()) {
                final Object o = getScriptableObject();
                if (o instanceof HTMLDocument) {
                    final Object e = elementWithFocus_.getScriptableObject();
                    if (e instanceof HTMLElement) {
                        ((HTMLDocument) o).setActiveElement((HTMLElement) e);
                    }
                }
            }
            elementWithFocus_.focus();
            elementWithFocus_.fireEvent(Event.TYPE_FOCUS);
            if (hasFeature(EVENT_FOCUS_FOCUS_IN_BLUR_OUT)) {
                elementWithFocus_.fireEvent(Event.TYPE_FOCUS_IN);
            }
        }

At line 2528 (https://github.com/HtmlUnit/htmlunit/blob/master/src/main/java/com/gargoylesoftware/htmlunit/html/HtmlPage.java#L2528) elementWithFocus_ could end up being null due to something that has been triggered in the fireEvent(Event.TYPE_FOCUS) method.

A couple of lines above the element is transfered to oldFocusedElement doing similar things (and that's also how I've fixed it in my setup when encountering the issue with jQuery + bunch of different frameworks).
Happy to follow up any ideas on how to get this reproduceable with vanilla JS :-)