var maskWidth = $(window).width();
alert(maskWidth);
I get 1263 from chrome and firefox (which is 1280 in fact, but with scroll it write a less value). IE print 1259.
How can I fix this problems with JQuery?
This isn't a problem with jQuery, it is returning the value of the displayable portion of your website.
You need to be looking at .innerWidth() here which will return results closer to which you require.
This method is not applicable to window and document objects; for these, use .width() instead.