Optimized blob view loading in repository

Changelog: changed
MR: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/65859

app/assets/javascripts/blob/viewer/index.js

@@ -6,6 +6,8 @@ import {
   REPO_BLOB_LOAD_VIEWER_START,
   REPO_BLOB_LOAD_VIEWER_FINISH,
   REPO_BLOB_LOAD_VIEWER,
+  REPO_BLOB_SWITCH_TO_VIEWER_START,
+  REPO_BLOB_SWITCH_VIEWER,
 } from '~/performance/constants';
 import { performanceMarkAndMeasure } from '~/performance/utils';
 import { fixTitle } from '~/tooltips';
@@ -49,6 +51,9 @@ export const handleBlobRichViewer = (viewer, type) => {
 
 export default class BlobViewer {
   constructor() {
+    performanceMarkAndMeasure({
+      mark: REPO_BLOB_LOAD_VIEWER_START,
+    });
     const viewer = document.querySelector('.blob-viewer[data-type="rich"]');
     const type = viewer?.dataset?.richType;
     BlobViewer.initAuxiliaryViewer();
@@ -141,7 +146,7 @@ export default class BlobViewer {
 
   switchToViewer(name) {
     performanceMarkAndMeasure({
-      mark: REPO_BLOB_LOAD_VIEWER_START,
+      mark: REPO_BLOB_SWITCH_TO_VIEWER_START,
     });
     const newViewer = this.$fileHolder[0].querySelector(`.blob-viewer[data-type='${name}']`);
     if (this.activeViewer === newViewer) return;
@@ -171,11 +176,15 @@ export default class BlobViewer {
     BlobViewer.loadViewer(newViewer)
       .then((viewer) => {
         $(viewer).renderGFM();
+        window.requestIdleCallback(() => {
+          this.$fileHolder.trigger('highlight:line');
+          handleLocationHash();
 
-        this.$fileHolder.trigger('highlight:line');
-        handleLocationHash();
+          viewer.setAttribute('data-loaded', 'true');
+          this.toggleCopyButtonState();
+          eventHub.$emit('showBlobInteractionZones', viewer.dataset.path);
+        });
 
-        this.toggleCopyButtonState();
         performanceMarkAndMeasure({
           mark: REPO_BLOB_LOAD_VIEWER_FINISH,
           measures: [
@@ -183,6 +192,10 @@ export default class BlobViewer {
               name: REPO_BLOB_LOAD_VIEWER,
               start: REPO_BLOB_LOAD_VIEWER_START,
             },
+            {
+              name: REPO_BLOB_SWITCH_VIEWER,
+              start: REPO_BLOB_SWITCH_TO_VIEWER_START,
+            },
           ],
         });
       })
@@ -205,9 +218,10 @@ export default class BlobViewer {
 
     return axios.get(url).then(({ data }) => {
       viewer.innerHTML = data.html;
-      viewer.setAttribute('data-loaded', 'true');
 
-      eventHub.$emit('showBlobInteractionZones', viewer.dataset.path);
+      window.requestIdleCallback(() => {
+        viewer.removeAttribute('data-loading');
+      });
 
       return viewer;
     });

app/assets/javascripts/performance/constants.js

@@ -83,7 +83,9 @@ export const PIPELINES_DETAIL_LINKS_JOB_RATIO = 'pipeline_graph_links_per_job_ra
 
 // Marks
 export const REPO_BLOB_LOAD_VIEWER_START = 'blobviewer-load-viewer-start';
+export const REPO_BLOB_SWITCH_TO_VIEWER_START = 'blobviewer-switch-to-viewerr-start';
 export const REPO_BLOB_LOAD_VIEWER_FINISH = 'blobviewer-load-viewer-finish';
 
 // Measures
-export const REPO_BLOB_LOAD_VIEWER = 'Repository File Viewer: loading the content';
+export const REPO_BLOB_LOAD_VIEWER = 'Repository File Viewer: loading the viewer';
+export const REPO_BLOB_SWITCH_VIEWER = 'Repository File Viewer: switching the viewer';

app/assets/stylesheets/framework/files.scss

@@ -508,3 +508,25 @@ span.idiff {
     }
   }
 }
+
+//
+// IMPORTANT PERFORMANCE OPTIMIZATION BELOW
+//
+// * :nth-of-type(1n+70) - makes sure we do not render lines 71+ right
+// away. Even though the HTML is injected in the DOM, as long as we do
+// not render those lines, the browser doesn't need to spend resources
+// calculating and repainting what's hidden.
+//
+// * :not(:last-of-type) makes sure that we output the last line of the
+// blob's snippet. This is important because the column with the line
+// numbers has auto width and is expanding based on the content in it.
+// This leads to unnecessary layout shift when the last lines of the
+// snippet are longer than two (2) digits.
+// EXAMPLE: Let's say, we have a blob with 100 lines. If we output 70
+// lines, and then, the remaining 30 (incl the line 100), it will lead
+// to the layout reflow and styles recalculation when we output line
+// 100 (because the width of '100' is always bigger than '70'). By
+// outputting the last line right away, we prevent that as the column
+// will always be expanded to the maximum needed width.
+.blob-viewer[data-loading] .file-content.code .line:nth-of-type(1n+70):not(:last-of-type),
+.blob-viewer[data-loading] .file-content.code .file-line-num:nth-of-type(1n+70):not(:last-of-type) {display: none !important;}

spec/frontend/blob/viewer/index_spec.js

@@ -6,6 +6,10 @@ import { setTestTimeout } from 'helpers/timeout';
 import BlobViewer from '~/blob/viewer/index';
 import axios from '~/lib/utils/axios_utils';
 
+const execImmediately = (callback) => {
+  callback();
+};
+
 describe('Blob viewer', () => {
   let blob;
   let mock;
@@ -17,6 +21,7 @@ describe('Blob viewer', () => {
   setTestTimeout(2000);
 
   beforeEach(() => {
+    jest.spyOn(window, 'requestIdleCallback').mockImplementation(execImmediately);
     $.fn.extend(jQueryMock);
     mock = new MockAdapter(axios);