Highlight selected file in the PR file tree (#23947) (#24126)

backport #23947 by @yusifeng 

before

![before](https://user-images.githubusercontent.com/36984894/230327904-6e712ca2-f777-4cad-99f3-53bc20008180.gif)

after

![after](https://user-images.githubusercontent.com/36984894/230327966-6e5dd971-f0df-427a-a80b-6a9b6db6065d.gif)

Co-authored-by: yusifeng <36984894+yusifeng@users.noreply.github.com>

web_src/js/components/DiffFileTree.vue

@@ -5,7 +5,7 @@
   >
     <!-- only render the tree if we're visible. in many cases this is something that doesn't change very often -->
     <div class="ui list">
-      <DiffFileTreeItem v-for="item in fileTree" :key="item.name" :item="item" />
+      <DiffFileTreeItem v-for="item in fileTree" :key="item.name" :item="item" :selected-file="selectedFile"/>
     </div>
     <div v-if="isIncomplete" id="diff-too-many-files-stats" class="gt-pt-2">
       <span class="gt-mr-2">{{ tooManyFilesMessage }}</span><a :class="['ui', 'basic', 'tiny', 'button', isLoadingNewData === true ? 'disabled' : '']" id="diff-show-more-files-stats" @click.stop="loadMoreData">{{ showMoreMessage }}</a>
@@ -25,7 +25,10 @@ export default {
   data: () => {
     const fileTreeIsVisible = localStorage.getItem(LOCAL_STORAGE_KEY) === 'true';
     pageData.diffFileInfo.fileTreeIsVisible = fileTreeIsVisible;
-    return pageData.diffFileInfo;
+    return {
+      ...pageData.diffFileInfo,
+      selectedFile: ''
+    };
   },
   computed: {
     fileTree() {
@@ -98,9 +101,16 @@ export default {
     pageData.diffFileInfo.files = this.files;
 
     document.querySelector('.diff-toggle-file-tree-button').addEventListener('click', this.toggleVisibility);
+
+    this.hashChangeListener = () => {
+      this.selectedFile = window.location.hash;
+    };
+    this.hashListener = window.addEventListener('hashchange', this.hashChangeListener);
+    this.selectedFile = window.location.hash;
   },
   unmounted() {
     document.querySelector('.diff-toggle-file-tree-button').removeEventListener('click', this.toggleVisibility);
+    window.removeEventListener('hashchange', this.hashChangeListener);
   },
   methods: {
     toggleVisibility() {

web_src/js/components/DiffFileTreeItem.vue

@@ -1,7 +1,7 @@
 <template>
   <div v-show="show" class="tooltip" :title="item.name">
     <!--title instead of tooltip above as the tooltip needs too much work with the current methods, i.e. not being loaded or staying open for "too long"-->
-    <div class="item" :class="item.isFile ? 'filewrapper gt-p-1' : ''">
+    <div class="item" :class="[item.isFile ? 'filewrapper gt-p-1 gt-ac' : '', selectedFile === genCompleteFileHash(item.file?.NameHash) ? 'selected' : '']">
       <!-- Files -->
       <SvgIcon
         v-if="item.isFile"
@@ -34,7 +34,7 @@
         <span class="gt-ellipsis">{{ item.name }}</span>
       </div>
       <div v-show="!collapsed">
-        <DiffFileTreeItem v-for="childItem in item.children" :key="childItem.name" :item="childItem" class="list" />
+        <DiffFileTreeItem v-for="childItem in item.children" :key="childItem.name" :item="childItem" class="list" :selected-file="selectedFile"/>
       </div>
     </div>
   </div>
@@ -54,6 +54,11 @@ export default {
       type: Boolean,
       required: false,
       default: true
+    },
+    selectedFile: {
+      type: String,
+      default: '',
+      required: true
     }
   },
   data: () => ({
@@ -76,6 +81,9 @@ export default {
       };
       return diffTypes[pType];
     },
+    genCompleteFileHash(hash) {
+      return `#diff-${hash}`;
+    }
   },
 };
 </script>
@@ -115,12 +123,18 @@ span.svg-icon.octicon-diff-renamed {
   padding-left: 18px !important;
 }
 
-.item.filewrapper:hover {
+.item.filewrapper:hover, div.directory:hover {
   color: var(--color-text);
   background: var(--color-hover);
   border-radius: 4px;
 }
 
+.item.filewrapper.selected {
+  color: var(--color-text);
+  background: var(--color-active);
+  border-radius: 4px;
+}
+
 div.directory {
   display: grid;
   grid-template-columns: 18px 20px auto;
@@ -128,12 +142,6 @@ div.directory {
   cursor: pointer;
 }
 
-div.directory:hover {
-  color: var(--color-text);
-  background: var(--color-hover);
-  border-radius: 4px;
-}
-
 div.list {
   padding-bottom: 0 !important;
   padding-top: inherit !important;