pathname and many other properties of location and links reflect only part of the URL:
http: //www.example.com/path/to/example.html?param1=2¶m3=4#fragment
^protocol^hostname ^pathname ^search ^hash
As you can see, the ?... part of the URL is not part of the pathname; it makes no sense to write a value containing ? to location.pathname, as that part of a URL cannot contain a question mark. Chrome is correcting your mistake by encoding the character to a sequence that means a literal question mark, which doesn’t terminate pathname.
These properties are great for breaking a URL into their constituent parts for you to process, but you probably don’t want to write to them in this case. Instead, write to location.href. This represents the whole URL, but it’s perfectly fine to write a relative URL to it; this will be worked out relative to the current value, so there is in fact no need to read and split the pathname at all:
location.href="https://stackoverflow.com/questions/3643041/myPage.xhtml?u="+encodeURIComponent(selected_user.Username);
Note the URL-encoding. If a username can contain characters other than alphanumerics you will probably need this to stop those characters breaking the parameter. Always URL-encode arbitrary strings before putting them into part of a URL.