When using Safari, often when logged-in and loading the edit page (or viewing articles) this error shows up in the console.
TypeError: undefined is not a function (evaluating 'performance.mark('mwLoadEnd')')
It is more often triggered on the first page load after localStorage.clear();.
The oddest part is that when this error happens, even way after the page is loaded, manually checking the performance global, it still shows performance.mark as undefined. So it's not a race condition, it's permanently gone.
However it's defined in the startup module. There is no way it can't run. And there is no error for performance.mark('mwLoadStart');, only for performance.mark('mwLoadEnd');.
