Thanks for taking the time to contribute to the jQuery project! Please provide a reduced test case on http://jsFiddle.net that reproduces the issue experienced to help us assess your ticket!

Additionally, test against the jQuery (edge) version to ensure the issue still exists.

You're right, some browsers will freeze when doing synchronous ajax request. That's why it's been said over and over again that doing synchronous requests is to be avoided at all cost.

In your case, it's not that the browser is frozen before beforeSend is called (lotsa before in there ;)), it's just that the synchronous request prevents the display reflow (which generally happens "sometimes later" than your actual DOM manipulation). I'm pretty confident that, if you try & call showLoader before the call to ajax rather than from inside the beforeSend callback, you'll end up with pretty much the same result.

There is nothing that can be done in jQuery to prevent this behaviour since it is not a jQuery bug (nor a browser one per se, seeing as synchronous xhr requests are supposed to freeze the browser anyway and that nothing says the browser has to carry on with the reflow in that instance).

All I can recommand is to NEVER EVER use synchronous requests. Asynchronous ones are easy enough to deal with.

here a good sln

http://blogs.microsoft.co.il/blogs/shlomo/archive/2012/05/06/jquery-ajax-with-async-set-to-false-and-beforesend-registration.aspx?utm_source=rss&utm_medium=rss&utm_campaign=jquery-ajax-with-async-set-to-false-and-beforesend-registration