Remove pointer events on the entire UI when a dropdown menu is open (#6648)

* Remove pointer events on the entire UI when a dropdown menu is open

This prevents operations to change the location of the menu such as
scrolling.

* Fix mistake from merge

app/javascript/mastodon/reducers/dropdown_menu.js

@@ -0,0 +1,18 @@
+import Immutable from 'immutable';
+import {
+  DROPDOWN_MENU_OPEN,
+  DROPDOWN_MENU_CLOSE,
+} from '../actions/dropdown_menu';
+
+const initialState = Immutable.Map({ openId: null, placement: null });
+
+export default function dropdownMenu(state = initialState, action) {
+  switch (action.type) {
+  case DROPDOWN_MENU_OPEN:
+    return state.merge({ openId: action.id, placement: action.placement });
+  case DROPDOWN_MENU_CLOSE:
+    return state.get('openId') === action.id ? state.set('openId', null) : state;
+  default:
+    return state;
+  }
+}

app/javascript/mastodon/reducers/index.js

@@ -1,4 +1,5 @@
 import { combineReducers } from 'redux-immutable';
+import dropdown_menu from './dropdown_menu';
 import timelines from './timelines';
 import meta from './meta';
 import alerts from './alerts';
@@ -26,6 +27,7 @@ import lists from './lists';
 import listEditor from './list_editor';
 
 const reducers = {
+  dropdown_menu,
   timelines,
   meta,
   alerts,