Work around regression in OO.ui.FloatableElement

FloatableElement incorrectly believes it doesn't need to
position itself when its closest scrollable is the same as
the closest scrollable of the container. This causes
the dotdotdot menu to be placed at the top left of the screen
if the notifications popup is not scrollable. If it is scrollable,
everything works fine. This regression was introduced by I5b5d7936c4d.

Work around this by setting overflow-x: auto; on the popup body.
This is a no-op, but tricks OOUI into thinking that the popup
body is scrollable even when it visually isn't. We can't use
overflow-y for this, because we still need to set overflow-y: auto;
on the popup body to work around a Chrome bug where right-floated
elements don't move over when the scrollbar appears.

Bug: T130153
Change-Id: I8bd8a26a4931444a760d2159779240272c51f966

modules/ooui/styles/mw.echo.ui.NotificationBadgeWidget.less

@@ -38,7 +38,9 @@
 
 			> .oo-ui-popupWidget-body {
 				// Work around scrollbar rendering bug in Chrome
-				overflow: visible;
+				overflow-y: visible;
+				// Work around OO.ui.FloatableElement bug (T130153)
+				overflow-x: auto;
 			}
 
 			> .oo-ui-popupWidget-footer {