Fix duplicate bubble race condition when hiding.

While a bubble is animating the hide, it's possible to show another
bubble within a bubble group. Then, when the hide animation completes,
`hideNow` would be called, clobbering the group state set by the newly
shown bubble.

r2/r2/public/static/js/ui.js

@@ -234,7 +234,7 @@ r.ui.Bubble = Backbone.View.extend({
 
     hideNow: function() {
         this.cancelTimeout()
-        if (this.options.group) {
+        if (this.options.group && this.options.group.current == this) {
             this.options.group.current = null
         }
         this.$el.hide()