When I call Sphinx login in my webcore app it doesn't display the login form and appears to be loading.
If I look at developer tools it pauses on e.querySelectorAll("\\f"), in Jquery 3.5.1 as if a breakpoint is set. Clicking continue and it runs as expected
I guess there is some leftover in your browser/development environment causing this?
Have you tried completely clearing the browser cache, and/or using a different browser to see if the behavior persists?