json - MVC4 Jquery / UI / Ajax Release Build breaks my form submissions? -

very strange issue.

i have mvc4 web application uses pop-up editing (through jqueryui , partial views) , works fine when application built in debug mode, or if it's built in release mode , run on iis express.

the httppost action either returns html partialview containing errors (which repopulates ui dialog box) or json response return json(new { success = true });

if deploy debug build iis, works fine.

however, if deploy release build iis stops working , client browsers (all of them) start treating json responses file downloads......?!

i've fiddled requests , different..... ones work send headers:

accept: */* content-type: application/x-www-form-urlencoded; charset=utf-8 x-requested-with: xmlhttprequest 

...and ones fail.....

accept: text/html, application/xhtml+xml, */* content-type: application/x-www-form-urlencoded 

...and no x-requested-with header.

tbh i've got no idea why request headers different based on build config....?! help?

well found out answer was......talk perfect storm....

i using jquery 1.9.1. apparently recent versions of jquery cause "bundling" break.....

in debug mode, , in release mode on iis express bundling was't doing anything. however, when published release-mode iis sprung life.....and promptly broke javascript.

....which preventing jquery.on handler overriding form submit button.....

....which causing form submitted via html , not ajax......

....which causing json response "downloaded file" browser......

....which makes me feel bit dumb asking question in first place clues there.....

the solution, of course, fix jquery*.min.js files don't break bundling (as described in attached question/answer). ho hum.