Allows for virtual scrolling to be disabled by a URL param

This sets a cookie as well so that future visits will also
be disabled.

app/assets/javascripts/diffs/index.js

@@ -2,6 +2,7 @@ import Cookies from 'js-cookie';
 import Vue from 'vue';
 import { mapActions, mapState, mapGetters } from 'vuex';
 import { parseBoolean } from '~/lib/utils/common_utils';
+import { getParameterValues } from '~/lib/utils/url_utility';
 import FindFile from '~/vue_shared/components/file_finder/index.vue';
 import eventHub from '../notes/event_hub';
 import diffsApp from './components/app.vue';
@@ -113,6 +114,11 @@ export default function initDiffsApp(store) {
           updateDatabase: false,
         });
       }
+
+      const vScrollingParam = getParameterValues('virtual_scrolling')[0];
+      if (vScrollingParam === 'false' || vScrollingParam === 'true') {
+        Cookies.set('diffs_virtual_scrolling', vScrollingParam);
+      }
     },
     methods: {
       ...mapActions('diffs', ['setRenderTreeList', 'setShowWhitespace']),

app/assets/javascripts/diffs/store/getters.js

+import Cookies from 'js-cookie';
 import { getParameterValues } from '~/lib/utils/url_utility';
 import { __, n__ } from '~/locale';
 import {
@@ -173,7 +174,16 @@ export function suggestionCommitMessage(state, _, rootState) {
     });
 }
 
-export const isVirtualScrollingEnabled = (state) =>
+export const isVirtualScrollingEnabled = (state) => {
+  const vSrollerCookie = Cookies.get('diffs_virtual_scrolling');
+
+  if (vSrollerCookie) {
+    return vSrollerCookie === 'true';
+  }
+
+  return (
     !state.viewDiffsFileByFile &&
     (window.gon?.features?.diffsVirtualScrolling ||
-    getParameterValues('virtual_scrolling')[0] === 'true');
+      getParameterValues('virtual_scrolling')[0] === 'true')
+  );
+};