Update dijit/popup::moveOffScreen() to explicitly [re]set style.left and style.right, to avoid a horizontal scrollbar in case left or right was previously set.   Also updating dijitOffScreen CSS class to use left: -9999px (or right: -9999px for RTL environments) instead of left:  50%, due to alleged problems with chrome's focus outline extending to the hidden element, and also to avoid a horizontal scrollbar in case the width of the offscreen element is >50% of the viewport width.   Fixes #16304 !strict.