JavaScript – onMouseMove doesn’t work with frameset

I’m creating a google chrome extension. I want to execute a function on each mousemove event (on window). So, of course, I set up extension, prepared everything, incjected the code into webpage (to avoid sandboxed environment), the actual code is like this:

window.addEventListener('mousemove', event => {
  myFunc(event, someOtherParams);
});

It worked very good so far. However, I just found a website where it doesn’t work. I tried to debug and inspect to find out why it doesn’t work. Simply, the onmousemove event is not being triggered. I noticed that document.body.tagName == 'FRAMESET' instead of 'BODY'. I didn’t even know that body may have different tag name. I searched on internet and found on MDN that body indeed may be a frameset. I’m pretty sure that it causes the issue.

The question is simple. On websites which use frameset instead of regular body, why doesn’t onmousemove event of window trigger when cursor is moved? How to set the event listener properly?

Thank you for your time.