Resolve "Lazy load images on the Frontend"

app/assets/javascripts/copy_as_gfm.js

 /* eslint-disable class-methods-use-this, object-shorthand, no-unused-vars, no-use-before-define, no-new, max-len, no-restricted-syntax, guard-for-in, no-continue */
 
 import './lib/utils/common_utils';
+import { placeholderImage } from './lazy_loader';
 
 const gfmRules = {
   // The filters referenced in lib/banzai/pipeline/gfm_pipeline.rb convert
@@ -56,6 +57,11 @@ const gfmRules = {
       return text;
     },
   },
+  ImageLazyLoadFilter: {
+    'img'(el, text) {
+      return `![${el.getAttribute('alt')}](${el.getAttribute('src')})`;
+    },
+  },
   VideoLinkFilter: {
     '.video-container'(el) {
       const videoEl = el.querySelector('video');
@@ -163,7 +169,9 @@ const gfmRules = {
       return text.trim().split('\n').map(s => `> ${s}`.trim()).join('\n');
     },
     'img'(el) {
-      return `![${el.getAttribute('alt')}](${el.getAttribute('src')})`;
+      const imageSrc = el.src;
+      const imageUrl = imageSrc && imageSrc !== placeholderImage ? imageSrc : (el.dataset.src || '');
+      return `![${el.getAttribute('alt')}](${imageUrl})`;
     },
     'a.anchor'(el, text) {
       // Don't render a Markdown link for the anchor link inside a heading

app/assets/javascripts/lazy_loader.js

+/* eslint-disable one-export, one-var, one-var-declaration-per-line */
+
+import _ from 'underscore';
+
+export const placeholderImage = 'data:image/gif;base64,R0lGODlhAQABAAAAACH5BAEKAAEALAAAAAABAAEAAAICTAEAOw==';
+const SCROLL_THRESHOLD = 300;
+
+export default class LazyLoader {
+  constructor(options = {}) {
+    this.lazyImages = [];
+    this.observerNode = options.observerNode || '#content-body';
+
+    const throttledScrollCheck = _.throttle(() => this.scrollCheck(), 300);
+    const debouncedElementsInView = _.debounce(() => this.checkElementsInView(), 300);
+
+    window.addEventListener('scroll', throttledScrollCheck);
+    window.addEventListener('resize', debouncedElementsInView);
+
+    const scrollContainer = options.scrollContainer || window;
+    scrollContainer.addEventListener('load', () => this.loadCheck());
+  }
+  searchLazyImages() {
+    this.lazyImages = [].slice.call(document.querySelectorAll('.lazy'));
+    this.checkElementsInView();
+  }
+  startContentObserver() {
+    const contentNode = document.querySelector(this.observerNode) || document.querySelector('body');
+
+    if (contentNode) {
+      const observer = new MutationObserver(() => this.searchLazyImages());
+
+      observer.observe(contentNode, {
+        childList: true,
+        subtree: true,
+      });
+    }
+  }
+  loadCheck() {
+    this.searchLazyImages();
+    this.startContentObserver();
+  }
+  scrollCheck() {
+    requestAnimationFrame(() => this.checkElementsInView());
+  }
+  checkElementsInView() {
+    const scrollTop = pageYOffset;
+    const visHeight = scrollTop + innerHeight + SCROLL_THRESHOLD;
+    let imgBoundRect, imgTop, imgBound;
+
+    // Loading Images which are in the current viewport or close to them
+    this.lazyImages = this.lazyImages.filter((selectedImage) => {
+      if (selectedImage.getAttribute('data-src')) {
+        imgBoundRect = selectedImage.getBoundingClientRect();
+
+        imgTop = scrollTop + imgBoundRect.top;
+        imgBound = imgTop + imgBoundRect.height;
+
+        if (scrollTop < imgBound && visHeight > imgTop) {
+          LazyLoader.loadImage(selectedImage);
+          return false;
+        }
+
+        return true;
+      }
+      return false;
+    });
+  }
+  static loadImage(img) {
+    if (img.getAttribute('data-src')) {
+      img.setAttribute('src', img.getAttribute('data-src'));
+      img.removeAttribute('data-src');
+      img.classList.remove('lazy');
+      img.classList.add('js-lazy-loaded');
+    }
+  }
+}

app/assets/javascripts/main.js

@@ -109,6 +109,7 @@ import './label_manager';
 import './labels';
 import './labels_select';
 import './layout_nav';
+import LazyLoader from './lazy_loader';
 import './line_highlighter';
 import './logo';
 import './member_expiration_date';
@@ -166,6 +167,11 @@ window.addEventListener('load', function onLoad() {
   gl.utils.handleLocationHash();
 }, false);
 
+gl.lazyLoader = new LazyLoader({
+  scrollContainer: window,
+  observerNode: '#content-body'
+});
+
 $(function () {
   var $body = $('body');
   var $document = $(document);

app/assets/stylesheets/framework/avatar.scss

@@ -35,6 +35,8 @@
   width: 40px;
   height: 40px;
   padding: 0;
+  background: $avatar-background;
+  overflow: hidden;
 
   &.avatar-inline {
     float: none;

app/assets/stylesheets/framework/typography.scss

@@ -11,8 +11,17 @@
   }
 
   img {
-    max-width: 100%;
+    /*max-width: 100%;*/
     margin: 0 0 8px;
+    min-width: 200px;
+    min-height: 100px;
+    background-color: $gray-lightest;
+  }
+
+  img.js-lazy-loaded {
+    min-width: none;
+    min-height: none;
+    background-color: none;
   }
 
   p a:not(.no-attachment-icon) img {

app/assets/stylesheets/framework/variables.scss

@@ -379,7 +379,9 @@ $issue-boards-card-shadow: rgba(186, 186, 186, 0.5);
 * Avatar
 */
 $avatar_radius: 50%;
-$avatar-border: $border-color;
+$avatar-border: $gray-normal;
+$avatar-border-hover: $gray-darker;
+$avatar-background: $gray-lightest;
 $gl-avatar-size: 40px;
 
 /*

app/helpers/avatars_helper.rb

@@ -11,17 +11,12 @@ module AvatarsHelper
   def user_avatar_without_link(options = {})
     avatar_size = options[:size] || 16
     user_name = options[:user].try(:name) || options[:user_name]
-    css_class = options[:css_class] || ''
     avatar_url = options[:url] || avatar_icon(options[:user] || options[:user_email], avatar_size)
     data_attributes = { container: 'body' }
 
-    if options[:lazy]
-      data_attributes[:src] = avatar_url
-    end
-
     image_tag(
-      options[:lazy] ? '' : avatar_url,
-      class: "avatar has-tooltip s#{avatar_size} #{css_class}",
+      avatar_url,
+      class: %W[avatar has-tooltip s#{avatar_size}].push(*options[:css_class]),
       alt: "#{user_name}'s avatar",
       title: user_name,
       data: data_attributes

app/helpers/emails_helper.rb

@@ -61,8 +61,8 @@ module EmailsHelper
     else
       image_tag(
         image_url('mailers/gitlab_header_logo.gif'),
-        size: "55x50",
-        alt: "GitLab"
+        size: '55x50',
+        alt: 'GitLab'
       )
     end
   end

app/helpers/lazy_image_tag_helper.rb

+module LazyImageTagHelper
+  def placeholder_image
+    "data:image/gif;base64,R0lGODlhAQABAAAAACH5BAEKAAEALAAAAAABAAEAAAICTAEAOw=="
+  end
+
+  # Override the default ActionView `image_tag` helper to support lazy-loading
+  def image_tag(source, options = {})
+    options = options.symbolize_keys
+
+    unless options.delete(:lazy) == false
+      options[:data] ||= {}
+      options[:data][:src] = path_to_image(source)
+      options[:class] ||= ""
+      options[:class] << " lazy"
+
+      source = placeholder_image
+    end
+
+    super(source, options)
+  end
+
+  # Required for Banzai::Filter::ImageLazyLoadFilter
+  module_function :placeholder_image
+end

app/helpers/version_check_helper.rb

@@ -2,7 +2,7 @@ module VersionCheckHelper
   def version_status_badge
     if Rails.env.production? && current_application_settings.version_check_enabled
       image_url = VersionCheck.new.url
-      image_tag image_url, class: 'js-version-status-badge'
+      image_tag image_url, class: 'js-version-status-badge', lazy: false
     end
   end
 end

app/models/concerns/cache_markdown_field.rb

@@ -11,7 +11,7 @@ module CacheMarkdownField
   extend ActiveSupport::Concern
 
   # Increment this number every time the renderer changes its output
-  CACHE_VERSION = 1
+  CACHE_VERSION = 2
 
   # changes to these attributes cause the cache to be invalidates
   INVALIDATED_BY = %w[author project].freeze

app/views/projects/blob/viewers/_image.html.haml

 .file-content.image_file
-  %img{ src: blob_raw_url, alt: viewer.blob.name }
+  %img{ 'data-src': blob_raw_url, alt: viewer.blob.name }

app/views/projects/diffs/viewers/_image.html.haml

@@ -8,7 +8,7 @@
   .image
     %span.wrap
       .frame{ class: (diff_file.deleted_file? ? 'deleted' : 'added') }
-        %img{ src: blob_raw_path, alt: diff_file.file_path }
+        %img{ 'data-src': blob_raw_path, alt: diff_file.file_path }
       %p.image-info= number_to_human_size(blob.size)
 - else
   .image
@@ -16,7 +16,7 @@
       %span.wrap
         .frame.deleted
           %a{ href: project_blob_path(@project, tree_join(diff_file.old_content_sha, diff_file.old_path)) }
-            %img{ src: old_blob_raw_path, alt: diff_file.old_path }
+            %img{ 'data-src': old_blob_raw_path, alt: diff_file.old_path }
         %p.image-info.hide
           %span.meta-filesize= number_to_human_size(old_blob.size)
           |
@@ -28,7 +28,7 @@
       %span.wrap
         .frame.added
           %a{ href: project_blob_path(@project, tree_join(diff_file.content_sha, diff_file.new_path)) }
-            %img{ src: blob_raw_path, alt: diff_file.new_path }
+            %img{ 'data-src': blob_raw_path, alt: diff_file.new_path }
         %p.image-info.hide
           %span.meta-filesize= number_to_human_size(blob.size)
           |
@@ -41,10 +41,10 @@
     .swipe.view.hide
       .swipe-frame
         .frame.deleted
-          %img{ src: old_blob_raw_path, alt: diff_file.old_path }
+          %img{ 'data-src': old_blob_raw_path, alt: diff_file.old_path }
         .swipe-wrap
           .frame.added
-            %img{ src: blob_raw_path, alt: diff_file.new_path }
+            %img{ 'data-src': blob_raw_path, alt: diff_file.new_path }
         %span.swipe-bar
           %span.top-handle
           %span.bottom-handle
@@ -52,9 +52,9 @@
     .onion-skin.view.hide
       .onion-skin-frame
         .frame.deleted
-          %img{ src: old_blob_raw_path, alt: diff_file.old_path }
+          %img{ 'data-src': old_blob_raw_path, alt: diff_file.old_path }
         .frame.added
-          %img{ src: blob_raw_path, alt: diff_file.new_path }
+          %img{ 'data-src': blob_raw_path, alt: diff_file.new_path }
         .controls
           .transparent
           .drag-track

changelogs/unreleased/34361-lazy-load-images-on-the-frontend.yml

+---
+title: Lazy load images for better Frontend performance
+merge_request: 12503
+author: 

doc/development/fe_guide/performance.md

@@ -23,6 +23,18 @@ controlled by the server.
 1. The backend code will most likely be using etags. You do not and should not check for status
 `304 Not Modified`. The browser will transform it for you.
 
+### Lazy Loading
+
+To improve the time to first render we are using lazy loading for images. This works by setting 
+the actual image source on the `data-src` attribute. After the HTML is rendered and JavaScript is loaded, 
+the value of `data-src` will be moved to `src` automatically if the image is in the current viewport.
+
+*  Prepare images in HTML for lazy loading by renaming the `src` attribute to `data-src`
+*  If you are using the Rails `image_tag` helper, all images will be lazy-loaded by default unless `lazy: false` is provided.
+
+If you are asynchronously adding content which contains lazy images then you need to call the function
+`gl.lazyLoader.searchLazyImages()` which will search for lazy images and load them if needed.
+
 ## Reducing Asset Footprint
 
 ### Page-specific JavaScript

features/steps/project/wiki.rb

@@ -114,7 +114,7 @@ class Spinach::Features::ProjectWiki < Spinach::FeatureSteps
   end
 
   step 'Image should be shown on the page' do
-    expect(page).to have_xpath("//img[@src=\"image.jpg\"]")
+    expect(page).to have_xpath("//img[@data-src=\"image.jpg\"]")
   end
 
   step 'I click on image link' do

lib/banzai/filter/gollum_tags_filter.rb

@@ -118,7 +118,7 @@ module Banzai
         end
 
         if path
-          content_tag(:img, nil, src: path, class: 'gfm')
+          content_tag(:img, nil, data: { src: path }, class: 'gfm')
         end
       end
 

lib/banzai/filter/image_lazy_load_filter.rb

+module Banzai
+  module Filter
+    # HTML filter that moves the value of the src attribute to the data-src attribute so it can be lazy loaded
+    class ImageLazyLoadFilter < HTML::Pipeline::Filter
+      def call
+        doc.xpath('descendant-or-self::img').each do |img|
+          img['class'] ||= '' << 'lazy'
+          img['data-src'] = img['src']
+          img['src'] = LazyImageTagHelper.placeholder_image          
+        end
+        
+        doc
+      end
+    end
+  end
+end

lib/banzai/filter/image_link_filter.rb

@@ -10,7 +10,7 @@ module Banzai
           link = doc.document.create_element(
             'a',
             class: 'no-attachment-icon',
-            href: img['src'],
+            href: img['data-src'] || img['src'],
             target: '_blank',
             rel: 'noopener noreferrer'
           )

lib/banzai/filter/relative_link_filter.rb

@@ -22,6 +22,7 @@ module Banzai
 
         doc.css('img, video').each do |el|
           process_link_attr el.attribute('src')
+          process_link_attr el.attribute('data-src')
         end
 
         doc

lib/banzai/pipeline/gfm_pipeline.rb

@@ -16,6 +16,7 @@ module Banzai
           Filter::MathFilter,
           Filter::UploadLinkFilter,
           Filter::VideoLinkFilter,
+          Filter::ImageLazyLoadFilter,
           Filter::ImageLinkFilter,
           Filter::EmojiFilter,
           Filter::TableOfContentsFilter,

spec/features/admin/admin_appearance_spec.rb

@@ -63,11 +63,11 @@ feature 'Admin Appearance', feature: true do
   end
 
   def logo_selector
-    '//img[@src^="/uploads/-/system/appearance/logo"]'
+    '//img[data-src^="/uploads/-/system/appearance/logo"]'
   end
 
   def header_logo_selector
-    '//img[@src^="/uploads/-/system/appearance/header_logo"]'
+    '//img[data-src^="/uploads/-/system/appearance/header_logo"]'
   end
 
   def logo_fixture

spec/features/markdown_spec.rb

@@ -100,7 +100,7 @@ describe 'GitLab Markdown', feature: true do
       end
 
       it 'permits img elements' do
-        expect(doc).to have_selector('img[src*="smile.png"]')
+        expect(doc).to have_selector('img[data-src*="smile.png"]')
       end
 
       it 'permits br elements' do

spec/features/uploads/user_uploads_avatar_to_group_spec.rb

@@ -18,7 +18,7 @@ feature 'User uploads avatar to group', feature: true do
 
     visit group_path(group)
 
-    expect(page).to have_selector(%Q(img[src$="/uploads/-/system/group/avatar/#{group.id}/dk.png"]))
+    expect(page).to have_selector(%Q(img[data-src$="/uploads/-/system/group/avatar/#{group.id}/dk.png"]))
 
     # Cheating here to verify something that isn't user-facing, but is important
     expect(group.reload.avatar.file).to exist

spec/features/uploads/user_uploads_avatar_to_profile_spec.rb

@@ -16,7 +16,7 @@ feature 'User uploads avatar to profile', feature: true do
 
     visit user_path(user)
 
-    expect(page).to have_selector(%Q(img[src$="/uploads/-/system/user/avatar/#{user.id}/dk.png"]))
+    expect(page).to have_selector(%Q(img[data-src$="/uploads/-/system/user/avatar/#{user.id}/dk.png"]))
 
     # Cheating here to verify something that isn't user-facing, but is important
     expect(user.reload.avatar.file).to exist

spec/helpers/application_helper_spec.rb

@@ -62,13 +62,13 @@ describe ApplicationHelper do
       avatar_url = "/uploads/-/system/project/avatar/#{project.id}/banana_sample.gif"
 
       expect(helper.project_icon(project.full_path).to_s)
-        .to eq "<img src=\"#{avatar_url}\" alt=\"Banana sample\" />"
+        .to eq "<img data-src=\"#{avatar_url}\" class=\" lazy\" src=\"#{LazyImageTagHelper.placeholder_image}\" />"
 
       allow(ActionController::Base).to receive(:asset_host).and_return(gitlab_host)
       avatar_url = "#{gitlab_host}/uploads/-/system/project/avatar/#{project.id}/banana_sample.gif"
 
       expect(helper.project_icon(project.full_path).to_s)
-        .to eq "<img src=\"#{avatar_url}\" alt=\"Banana sample\" />"
+        .to eq "<img data-src=\"#{avatar_url}\" class=\" lazy\" src=\"#{LazyImageTagHelper.placeholder_image}\" />"
     end
 
     it 'gives uploaded icon when present' do
@@ -77,7 +77,8 @@ describe ApplicationHelper do
       allow_any_instance_of(Project).to receive(:avatar_in_git).and_return(true)
 
       avatar_url = "#{gitlab_host}#{project_avatar_path(project)}"
-      expect(helper.project_icon(project.full_path).to_s).to match(image_tag(avatar_url))
+      expect(helper.project_icon(project.full_path).to_s)
+        .to eq "<img data-src=\"#{avatar_url}\" class=\" lazy\" src=\"#{LazyImageTagHelper.placeholder_image}\" />"
     end
   end
 

spec/helpers/avatars_helper_spec.rb

@@ -27,11 +27,11 @@ describe AvatarsHelper do
 
     it 'displays user avatar' do
       is_expected.to eq image_tag(
-        avatar_icon(user, 16),
-        class: 'avatar has-tooltip s16 ',
+        LazyImageTagHelper.placeholder_image,
+        class: 'avatar has-tooltip s16  lazy',
         alt: "#{user.name}'s avatar",
         title: user.name,
-        data: { container: 'body' }
+        data: { container: 'body', src: avatar_icon(user, 16) }
       )
     end
 
@@ -40,22 +40,8 @@ describe AvatarsHelper do
 
       it 'uses provided css_class' do
         is_expected.to eq image_tag(
-          avatar_icon(user, 16),
-          class: "avatar has-tooltip s16 #{options[:css_class]}",
-          alt: "#{user.name}'s avatar",
-          title: user.name,
-          data: { container: 'body' }
-        )
-      end
-    end
-
-    context 'with lazy parameter' do
-      let(:options) { { user: user, lazy: true } }
-
-      it 'uses data-src instead of src' do
-        is_expected.to eq image_tag(
-          '',
-          class: 'avatar has-tooltip s16 ',
+          LazyImageTagHelper.placeholder_image,
+          class: "avatar has-tooltip s16 #{options[:css_class]}  lazy",
           alt: "#{user.name}'s avatar",
           title: user.name,
           data: { container: 'body', src: avatar_icon(user, 16) }
@@ -68,11 +54,11 @@ describe AvatarsHelper do
 
       it 'uses provided size' do
         is_expected.to eq image_tag(
-          avatar_icon(user, options[:size]),
-          class: "avatar has-tooltip s#{options[:size]} ",
+          LazyImageTagHelper.placeholder_image,
+          class: "avatar has-tooltip s#{options[:size]}  lazy",
           alt: "#{user.name}'s avatar",
           title: user.name,
-          data: { container: 'body' }
+          data: { container: 'body', src: avatar_icon(user, options[:size]) }
         )
       end
     end
@@ -82,11 +68,11 @@ describe AvatarsHelper do
 
       it 'uses provided url' do
         is_expected.to eq image_tag(
-          options[:url],
-          class: 'avatar has-tooltip s16 ',
+          LazyImageTagHelper.placeholder_image,
+          class: 'avatar has-tooltip s16  lazy',
           alt: "#{user.name}'s avatar",
           title: user.name,
-          data: { container: 'body' }
+          data: { container: 'body', src: options[:url] }
         )
       end
     end
@@ -99,22 +85,22 @@ describe AvatarsHelper do
 
         it 'prefers user parameter' do
           is_expected.to eq image_tag(
-            avatar_icon(user, 16),
-            class: 'avatar has-tooltip s16 ',
+            LazyImageTagHelper.placeholder_image,
+            class: 'avatar has-tooltip s16  lazy',
             alt: "#{user.name}'s avatar",
             title: user.name,
-            data: { container: 'body' }
+            data: { container: 'body', src: avatar_icon(user, 16) }
           )
         end
       end
 
       it 'uses user_name and user_email parameter if user is not present' do
         is_expected.to eq image_tag(
-          avatar_icon(options[:user_email], 16),
-          class: 'avatar has-tooltip s16 ',
+          LazyImageTagHelper.placeholder_image,
+          class: 'avatar has-tooltip s16  lazy',
           alt: "#{options[:user_name]}'s avatar",
           title: options[:user_name],
-          data: { container: 'body' }
+          data: { container: 'body', src: avatar_icon(options[:user_email], 16) }
         )
       end
     end

spec/javascripts/lazy_loader_spec.js

+import LazyLoader from '~/lazy_loader';
+
+let lazyLoader = null;
+
+describe('LazyLoader', function () {
+  preloadFixtures('issues/issue_with_comment.html.raw');
+
+  beforeEach(function () {
+    loadFixtures('issues/issue_with_comment.html.raw');
+    lazyLoader = new LazyLoader({
+      observerNode: 'body',
+    });
+    // Doing everything that happens normally in onload
+    lazyLoader.loadCheck();
+  });
+  describe('behavior', function () {
+    it('should copy value from data-src to src for img 1', function (done) {
+      const img = document.querySelectorAll('img[data-src]')[0];
+      const originalDataSrc = img.getAttribute('data-src');
+      img.scrollIntoView();
+
+      setTimeout(() => {
+        expect(img.getAttribute('src')).toBe(originalDataSrc);
+        expect(document.getElementsByClassName('js-lazy-loaded').length).toBeGreaterThan(0);
+        done();
+      }, 100);
+    });
+
+    it('should lazy load dynamically added data-src images', function (done) {
+      const newImg = document.createElement('img');
+      const testPath = '/img/testimg.png';
+      newImg.className = 'lazy';
+      newImg.setAttribute('data-src', testPath);
+      document.body.appendChild(newImg);
+      newImg.scrollIntoView();
+
+      setTimeout(() => {
+        expect(newImg.getAttribute('src')).toBe(testPath);
+        expect(document.getElementsByClassName('js-lazy-loaded').length).toBeGreaterThan(0);
+        done();
+      }, 100);
+    });
+
+    it('should not alter normal images', function (done) {
+      const newImg = document.createElement('img');
+      const testPath = '/img/testimg.png';
+      newImg.setAttribute('src', testPath);
+      document.body.appendChild(newImg);
+      newImg.scrollIntoView();
+
+      setTimeout(() => {
+        expect(newImg).not.toHaveClass('js-lazy-loaded');
+        done();
+      }, 100);
+    });
+  });
+});

spec/lib/banzai/filter/gollum_tags_filter_spec.rb

@@ -22,7 +22,7 @@ describe Banzai::Filter::GollumTagsFilter, lib: true do
       tag = '[[images/image.jpg]]'
       doc = filter("See #{tag}", project_wiki: project_wiki)
 
-      expect(doc.at_css('img')['src']).to eq "#{project_wiki.wiki_base_path}/images/image.jpg"
+      expect(doc.at_css('img')['data-src']).to eq "#{project_wiki.wiki_base_path}/images/image.jpg"
     end
 
     it 'does not creates img tag if image does not exist' do
@@ -40,7 +40,7 @@ describe Banzai::Filter::GollumTagsFilter, lib: true do
       tag = '[[http://example.com/image.jpg]]'
       doc = filter("See #{tag}", project_wiki: project_wiki)
 
-      expect(doc.at_css('img')['src']).to eq "http://example.com/image.jpg"
+      expect(doc.at_css('img')['data-src']).to eq "http://example.com/image.jpg"
     end
 
     it 'does not creates img tag for invalid URL' do

spec/lib/banzai/filter/image_lazy_load_filter_spec.rb

+require 'spec_helper'
+
+describe Banzai::Filter::ImageLazyLoadFilter, lib: true do
+  include FilterSpecHelper
+
+  def image(path)
+    %(<img src="#{path}" />)
+  end
+
+  it 'transforms the image src to a data-src' do
+    doc = filter(image('/uploads/e90decf88d8f96fe9e1389afc2e4a91f/test.jpg'))
+    expect(doc.at_css('img')['data-src']).to eq '/uploads/e90decf88d8f96fe9e1389afc2e4a91f/test.jpg'
+  end
+
+  it 'works with external images' do
+    doc = filter(image('https://i.imgur.com/DfssX9C.jpg'))
+    expect(doc.at_css('img')['data-src']).to eq 'https://i.imgur.com/DfssX9C.jpg'
+  end
+end

spec/support/matchers/markdown_matchers.rb

@@ -17,7 +17,7 @@ module MarkdownMatchers
       image = actual.at_css('img[alt="Relative Image"]')
 
       expect(link['href']).to end_with('master/doc/README.md')
-      expect(image['src']).to end_with('master/app/assets/images/touch-icon-ipad.png')
+      expect(image['data-src']).to end_with('master/app/assets/images/touch-icon-ipad.png')
     end
   end
 
@@ -70,7 +70,7 @@ module MarkdownMatchers
   # GollumTagsFilter
   matcher :parse_gollum_tags do
     def have_image(src)
-      have_css("img[src$='#{src}']")
+      have_css("img[data-src$='#{src}']")
     end
 
     prefix = '/namespace1/gitlabhq/wikis'