Discussion filters - ensured note links resolves to the right note

app/assets/javascripts/notes/components/discussion_filter.vue

 <script>
 import $ from 'jquery';
 import { mapGetters, mapActions } from 'vuex';
+import { getLocationHash } from '../../lib/utils/url_utility';
 import Icon from '~/vue_shared/components/icon.vue';
 import {
   DISCUSSION_FILTERS_DEFAULT_VALUE,
@@ -44,29 +45,47 @@ export default {
       eventHub.$on('MergeRequestTabChange', this.toggleFilters);
       this.toggleFilters(currentTab);
     }
+
+    window.addEventListener('hashchange', this.handleLocationHash);
+    this.handleLocationHash();
   },
   mounted() {
     this.toggleCommentsForm();
   },
+  destroyed() {
+    window.removeEventListener('hashchange', this.handleLocationHash);
+  },
   methods: {
-    ...mapActions(['filterDiscussion', 'setCommentsDisabled']),
+    ...mapActions(['filterDiscussion', 'setCommentsDisabled', 'setTargetNoteHash']),
     selectFilter(value) {
       const filter = parseInt(value, 10);
 
       // close dropdown
-      $(this.$refs.dropdownToggle).dropdown('toggle');
+      this.toggleDropdown();
 
       if (filter === this.currentValue) return;
       this.currentValue = filter;
       this.filterDiscussion({ path: this.getNotesDataByProp('discussionsPath'), filter });
       this.toggleCommentsForm();
     },
+    toggleDropdown() {
+      $(this.$refs.dropdownToggle).dropdown('toggle');
+    },
     toggleCommentsForm() {
       this.setCommentsDisabled(this.currentValue === HISTORY_ONLY_FILTER_VALUE);
     },
     toggleFilters(tab) {
       this.displayFilters = tab === DISCUSSION_TAB_LABEL;
     },
+    handleLocationHash() {
+      const hash = getLocationHash();
+
+      if (/^note_/.test(hash) && this.currentValue !== DISCUSSION_FILTERS_DEFAULT_VALUE) {
+        this.selectFilter(this.defaultValue);
+        this.toggleDropdown(); // close dropdown
+        this.setTargetNoteHash(hash);
+      }
+    },
   },
 };
 </script>

changelogs/unreleased/54484-anchor-links-to-comments-or-system-notes-can-break-with-discussion-filters.yml

+---
+title: Ensured links to a comment or system note anchor resolves to the right note if a user has a discussion filter.
+merge_request: 24228
+author:
+type: changed

spec/javascripts/notes/components/discussion_filter_spec.js

 import Vue from 'vue';
 import createStore from '~/notes/stores';
 import DiscussionFilter from '~/notes/components/discussion_filter.vue';
+import { DISCUSSION_FILTERS_DEFAULT_VALUE } from '~/notes/constants';
 import { mountComponentWithStore } from '../../helpers/vue_mount_component_helper';
 import { discussionFiltersMock, discussionMock } from '../mock_data';
 
@@ -20,16 +21,14 @@ describe('DiscussionFilter component', () => {
       },
     ];
     const Component = Vue.extend(DiscussionFilter);
-    const selectedValue = discussionFiltersMock[0].value;
+    const selectedValue = DISCUSSION_FILTERS_DEFAULT_VALUE;
+    const props = { filters: discussionFiltersMock, selectedValue };
 
     store.state.discussions = discussions;
     return mountComponentWithStore(Component, {
       el: null,
       store,
-      props: {
-        filters: discussionFiltersMock,
-        selectedValue,
-      },
+      props,
     });
   };
 
@@ -115,4 +114,41 @@ describe('DiscussionFilter component', () => {
       });
     });
   });
+
+  describe('URL with Links to notes', () => {
+    afterEach(() => {
+      window.location.hash = '';
+    });
+
+    it('updates the filter when the URL links to a note', done => {
+      window.location.hash = `note_${discussionMock.notes[0].id}`;
+      vm.currentValue = discussionFiltersMock[2].value;
+      vm.handleLocationHash();
+
+      vm.$nextTick(() => {
+        expect(vm.currentValue).toEqual(DISCUSSION_FILTERS_DEFAULT_VALUE);
+        done();
+      });
+    });
+
+    it('does not update the filter when the current filter is "Show all activity"', done => {
+      window.location.hash = `note_${discussionMock.notes[0].id}`;
+      vm.handleLocationHash();
+
+      vm.$nextTick(() => {
+        expect(vm.currentValue).toEqual(DISCUSSION_FILTERS_DEFAULT_VALUE);
+        done();
+      });
+    });
+
+    it('only updates filter when the URL links to a note', done => {
+      window.location.hash = `testing123`;
+      vm.handleLocationHash();
+
+      vm.$nextTick(() => {
+        expect(vm.currentValue).toEqual(DISCUSSION_FILTERS_DEFAULT_VALUE);
+        done();
+      });
+    });
+  });
 });