Step to reproduce:

1. Try to open non-exist page under dev environment, you see collapsed stack trace. It's OK.
2. Try to open non-exist page under test environment (toolbar should be disabled, profiler: collect: false OR web_profiler: toolbar: false). You see uncollapsed stack trace and infinite loading page.

I installed the symfony-demo project and reproduce the issue too.

Some of the browsers display a pop-up window:

A script on this page may be busy, or it may have stopped responding. You can stop the script now, open the script in the debugger, or let the script continue.

              // Prevents from disallowing clicks on links inside toggles /
                    var toggleLinks = document.querySelectorAll('.sf-toggle a');
                    for (var i = 0; i < toggleLinks.length; i++) {
                        addEventListener(toggleLinks[i], 'click', function(e) {
                            e.stopPropagation();
                        });
                    }

Regards,
Dmitry