Some things block some of your viewport, such a history/bookmarks side panel. This includes the taskbar, unless it's on Autohide

Then window.screen.width != window.screen.availWidth. :)

If you are drawing things to the viewport, you ought to use availWidth / availHeight to determine if things are off the viewport etc. Be sure to recalculate on window resize otherwise you won't know when the sidepanel has been closed.

Note that not all of the width given by this property may be available to the window itself. When other widgets occupy space that cannot be used by the window object, there is a difference in window.screen.width and window.screen.availWidth.

Source.