Closed:
Open:
Note that there is an extra right margin on the whole page when the article dropdown is open (although this will vary by OS, as it matches scrollbar width).
There is also an invisible click blocker covering the rest of the page, so although nothing is styled as disabled, clicking on anything outside the dropdown (e.g. on the user menu, or the notifications icon) will just result in the article selector closing.
Modal click blockers should only be used with modal popups and appropriate styling. To ensure the article selector is closed when clicking elsewhere we can bind a mousedown handler to the document, which is standard practice in OOUI.