svermillion/jquery-ui — comment on commit 12e23e291282b849d1cbd67b50be2f71b15871fa
File:  line 0
By svermillion:

The absolute positioning is specified in the core theme (jquery-ui/themes/base/jquery.ui.core.css: .ui-widget-overlay), but those without that css will see the overlay in the wrong place (as described in ticket). Makes sense to always have the absolute 0, 0 positioning for an overlay.