DevNet pages width
For now there are 2 layouts of pages (by stretch attribute). One is used at mainpage,memberlist,profile (maybe some other pages) and is not stretchable, The orher one is used at all other pages and is stretchable. I understand that changing main page to be stretchable is something not trivial and maybe not needed. But all other pages I think should be stretchable.
Well if you’re making it slimmer (not wider), why not also make the sidebar smaller or just remove it completely if needed. Put important info on top of screen. Also, you could make the poster name, picture and links smaller. Turn it more in to a column.
Heck, you’d end up with a mobile page :P. m.developer.qt.nokia.com?
Expanding to the width of the browser is a good thing. Sometimes there are contents that can better be viewed with a broad browser.
I don’t know what this is related to ultra wide screens, other than that you browser window is in maximize mode. That would be nothing a web designer should be concerned of.