Move syntax highlighting to web worker (#11017)

This should eliminate page freezes when loading big files/diff.
`highlightBlock` is needed to preserve existing nodes when highlighting
and for that, highlight.js needs access to the DOM API so I added a DOM
implementation to make it work, which adds around 300kB to the output
file size of the lazy-loaded `highlight.js`.

Co-authored-by: Lauris BH <lauris@nix.lv>

3 files changed, 34 insertions, 16 deletions

diff --git a/web_src/js/features/highlight.js b/web_src/js/features/highlight.js
index dcd8a8d21e..d3f6ba71b8 100644
--- a/web_src/js/features/highlight.js
+++ b/web_src/js/features/highlight.js
@@ -1,12 +1,19 @@
-export default async function initHighlight() {
-  if (!window.config || !window.config.HighlightJS) return;
+export default async function highlight(elementOrNodeList) {
+  if (!window.config || !window.config.HighlightJS || !elementOrNodeList) return;
+  const nodes = 'length' in elementOrNodeList ? elementOrNodeList : [elementOrNodeList];
+  if (!nodes.length) return;
 
-  const hljs = await import(/* webpackChunkName: "highlight" */'highlight.js');
+  const {default: Worker} = await import(/* webpackChunkName: "highlight" */'./highlight.worker.js');
+  const worker = new Worker();
 
-  const nodes = [].slice.call(document.querySelectorAll('pre code') || []);
-  for (let i = 0; i < nodes.length; i++) {
-    hljs.highlightBlock(nodes[i]);
-  }
+  worker.addEventListener('message', ({data}) => {
+    const {index, html} = data;
+    nodes[index].outerHTML = html;
+  });
 
-  return hljs;
+  for (let index = 0; index < nodes.length; index++) {
+    const node = nodes[index];
+    if (!node) continue;
+    worker.postMessage({index, html: node.outerHTML});
+  }
 }
diff --git a/web_src/js/features/highlight.worker.js b/web_src/js/features/highlight.worker.js
new file mode 100644
index 0000000000..7d6cc4e438
--- /dev/null
+++ b/web_src/js/features/highlight.worker.js
@@ -0,0 +1,12 @@
+import {highlightBlock} from 'highlight.js';
+import {createWindow} from 'domino';
+
+self.onmessage = function ({data}) {
+  const window = createWindow();
+  self.document = window.document;
+
+  const {index, html} = data;
+  document.body.innerHTML = html;
+  highlightBlock(document.body.firstChild);
+  self.postMessage({index, html: document.body.innerHTML});
+};
diff --git a/web_src/js/index.js b/web_src/js/index.js
index 6476b2cfbf..63a5134bbd 100644
--- a/web_src/js/index.js
+++ b/web_src/js/index.js
@@ -11,12 +11,12 @@ import './vendor/semanticdropdown.js';
 import {svg} from './utils.js';
 
 import initContextPopups from './features/contextpopup.js';
-import initHighlight from './features/highlight.js';
 import initGitGraph from './features/gitgraph.js';
 import initClipboard from './features/clipboard.js';
 import initUserHeatmap from './features/userheatmap.js';
 import initDateTimePicker from './features/datetimepicker.js';
 import createDropzone from './features/dropzone.js';
+import highlight from './features/highlight.js';
 import ActivityTopAuthors from './components/ActivityTopAuthors.vue';
 
 const {AppSubUrl, StaticUrlPrefix, csrf} = window.config;
@@ -29,7 +29,6 @@ let previewFileModes;
 let simpleMDEditor;
 const commentMDEditors = {};
 let codeMirrorEditor;
-let hljs;
 
 // Silence fomantic's error logging when tabs are used without a target content element
 $.fn.tab.settings.silent = true;
@@ -49,7 +48,7 @@ function initCommentPreviewTab($form) {
       $previewPanel.html(data);
       emojify.run($previewPanel[0]);
       $('pre code', $previewPanel[0]).each(function () {
-        hljs.highlightBlock(this);
+        highlight(this);
       });
     });
   });
@@ -75,7 +74,7 @@ function initEditPreviewTab($form) {
         $previewPanel.html(data);
         emojify.run($previewPanel[0]);
         $('pre code', $previewPanel[0]).each(function () {
-          hljs.highlightBlock(this);
+          highlight(this);
         });
       });
     });
@@ -1011,7 +1010,7 @@ async function initRepository() {
               $renderContent.html(data.content);
               emojify.run($renderContent[0]);
               $('pre code', $renderContent[0]).each(function () {
-                hljs.highlightBlock(this);
+                highlight(this);
               });
             }
             const $content = $segment.parent();
@@ -1337,7 +1336,7 @@ function initWikiForm() {
               preview.innerHTML = `<div class="markdown ui segment">${data}</div>`;
               emojify.run($('.editor-preview')[0]);
               $(preview).find('pre code').each((_, e) => {
-                hljs.highlightBlock(e);
+                highlight(e);
               });
             });
           };
@@ -2633,8 +2632,8 @@ $(document).ready(async () => {
   });
 
   // parallel init of lazy-loaded features
-  [hljs] = await Promise.all([
-    initHighlight(),
+  await Promise.all([
+    highlight(document.querySelectorAll('pre code')),
     initGitGraph(),
     initClipboard(),
     initUserHeatmap(),