Merge branch...

Merge branch '33183-request-inline-or-parallel-diffs-instead-fetching-all-data-from-backend-in-one-request' into 'master'

MR Diffs | Load Diff Types Lazily

Closes #39494

See merge request gitlab-org/gitlab!19930

app/assets/javascripts/diffs/components/app.vue

@@ -6,6 +6,7 @@ import { __ } from '~/locale';
 import createFlash from '~/flash';
 import PanelResizer from '~/vue_shared/components/panel_resizer.vue';
 import glFeatureFlagsMixin from '~/vue_shared/mixins/gl_feature_flags_mixin';
+import { isSingleViewStyle } from '~/helpers/diffs_helper';
 import eventHub from '../../notes/event_hub';
 import CompareVersions from './compare_versions.vue';
 import DiffFile from './diff_file.vue';
@@ -145,6 +146,9 @@ export default {
   },
   watch: {
     diffViewType() {
+      if (this.needsReload() || this.needsFirstLoad()) {
+        this.refetchDiffData();
+      }
       this.adjustView();
     },
     shouldShow() {
@@ -224,6 +228,16 @@ export default {
         { timeout: 1000 },
       );
     },
+    needsReload() {
+      return (
+        this.glFeatures.singleMrDiffView &&
+        this.diffFiles.length &&
+        isSingleViewStyle(this.diffFiles[0])
+      );
+    },
+    needsFirstLoad() {
+      return this.glFeatures.singleMrDiffView && !this.diffFiles.length;
+    },
     fetchData(toggleTree = true) {
       if (this.glFeatures.diffsBatchLoad) {
         this.fetchDiffFilesMeta()
@@ -237,6 +251,13 @@ export default {
           });
 
         this.fetchDiffFilesBatch()
+          .then(() => {
+            // Guarantee the discussions are assigned after the batch finishes.
+            // Just watching the length of the discussions or the diff files
+            // isn't enough, because with split diff loading, neither will
+            // change when loading the other half of the diff files.
+            this.setDiscussions();
+          })
           .then(() => this.startDiffRendering())
           .catch(() => {
             createFlash(__('Something went wrong on our end. Please try again!'));
@@ -250,6 +271,7 @@ export default {
 
             requestIdleCallback(
               () => {
+                this.setDiscussions();
                 this.startRenderDiffsQueue();
               },
               { timeout: 1000 },

app/assets/javascripts/diffs/components/diff_file.vue

@@ -4,6 +4,7 @@ import _ from 'underscore';
 import { GlLoadingIcon } from '@gitlab/ui';
 import { __, sprintf } from '~/locale';
 import createFlash from '~/flash';
+import { hasDiff } from '~/helpers/diffs_helper';
 import eventHub from '../../notes/event_hub';
 import DiffFileHeader from './diff_file_header.vue';
 import DiffContent from './diff_content.vue';
@@ -55,12 +56,7 @@ export default {
       return this.isLoadingCollapsedDiff || (!this.file.renderIt && !this.isCollapsed);
     },
     hasDiff() {
-      return (
-        (this.file.highlighted_diff_lines &&
-          this.file.parallel_diff_lines &&
-          this.file.parallel_diff_lines.length > 0) ||
-        !this.file.blob.readable_text
-      );
+      return hasDiff(this.file);
     },
     isFileTooLarge() {
       return this.file.viewer.error === diffViewerErrors.too_large;

app/assets/javascripts/diffs/store/actions.js

@@ -65,6 +65,10 @@ export const fetchDiffFiles = ({ state, commit }) => {
     w: state.showWhitespace ? '0' : '1',
   };
 
+  if (state.useSingleDiffStyle) {
+    urlParams.view = state.diffViewType;
+  }
+
   commit(types.SET_LOADING, true);
 
   worker.addEventListener('message', ({ data }) => {
@@ -90,13 +94,22 @@ export const fetchDiffFiles = ({ state, commit }) => {
 };
 
 export const fetchDiffFilesBatch = ({ commit, state }) => {
+  const urlParams = {
+    per_page: DIFFS_PER_PAGE,
+    w: state.showWhitespace ? '0' : '1',
+  };
+
+  if (state.useSingleDiffStyle) {
+    urlParams.view = state.diffViewType;
+  }
+
   commit(types.SET_BATCH_LOADING, true);
   commit(types.SET_RETRIEVING_BATCHES, true);
 
   const getBatch = page =>
     axios
       .get(state.endpointBatch, {
-        params: { page, per_page: DIFFS_PER_PAGE, w: state.showWhitespace ? '0' : '1' },
+        params: { ...urlParams, page },
       })
       .then(({ data: { pagination, diff_files } }) => {
         commit(types.SET_DIFF_DATA_BATCH, { diff_files });
@@ -150,7 +163,10 @@ export const assignDiscussionsToDiff = (
   { commit, state, rootState },
   discussions = rootState.notes.discussions,
 ) => {
-  const diffPositionByLineCode = getDiffPositionByLineCode(state.diffFiles);
+  const diffPositionByLineCode = getDiffPositionByLineCode(
+    state.diffFiles,
+    state.useSingleDiffStyle,
+  );
   const hash = getLocationHash();
 
   discussions
@@ -339,24 +355,23 @@ export const toggleFileDiscussions = ({ getters, dispatch }, diff) => {
 
 export const toggleFileDiscussionWrappers = ({ commit }, diff) => {
   const discussionWrappersExpanded = allDiscussionWrappersExpanded(diff);
-  let linesWithDiscussions;
-  if (diff.highlighted_diff_lines) {
-    linesWithDiscussions = diff.highlighted_diff_lines.filter(line => line.discussions.length);
-  }
-  if (diff.parallel_diff_lines) {
-    linesWithDiscussions = diff.parallel_diff_lines.filter(
-      line =>
-        (line.left && line.left.discussions.length) ||
-        (line.right && line.right.discussions.length),
+  const lineCodesWithDiscussions = new Set();
+  const { parallel_diff_lines: parallelLines, highlighted_diff_lines: inlineLines } = diff;
+  const allLines = inlineLines.concat(
+    parallelLines.map(line => line.left),
+    parallelLines.map(line => line.right),
   );
-  }
+  const lineHasDiscussion = line => Boolean(line?.discussions.length);
+  const registerDiscussionLine = line => lineCodesWithDiscussions.add(line.line_code);
+
+  allLines.filter(lineHasDiscussion).forEach(registerDiscussionLine);
 
-  if (linesWithDiscussions.length) {
-    linesWithDiscussions.forEach(line => {
+  if (lineCodesWithDiscussions.size) {
+    Array.from(lineCodesWithDiscussions).forEach(lineCode => {
       commit(types.TOGGLE_LINE_DISCUSSIONS, {
         fileHash: diff.file_hash,
-        lineCode: line.line_code,
         expanded: !discussionWrappersExpanded,
+        lineCode,
       });
     });
   }

app/assets/javascripts/diffs/store/mutations.js

@@ -45,26 +45,28 @@ export default {
   },
 
   [types.SET_DIFF_DATA](state, data) {
+    let files = state.diffFiles;
+
     if (
-      !(
-        gon &&
-        gon.features &&
-        gon.features.diffsBatchLoad &&
-        window.location.search.indexOf('diff_id') === -1
-      )
+      !(gon?.features?.diffsBatchLoad && window.location.search.indexOf('diff_id') === -1) &&
+      data.diff_files
     ) {
-      prepareDiffData(data);
+      files = prepareDiffData(data, files);
     }
 
     Object.assign(state, {
       ...convertObjectPropsToCamelCase(data),
+      diffFiles: files,
     });
   },
 
   [types.SET_DIFF_DATA_BATCH](state, data) {
-    prepareDiffData(data);
+    const files = prepareDiffData(data, state.diffFiles);
 
-    state.diffFiles.push(...data.diff_files);
+    Object.assign(state, {
+      ...convertObjectPropsToCamelCase(data),
+      diffFiles: files,
+    });
   },
 
   [types.RENDER_FILE](state, file) {
@@ -88,11 +90,11 @@ export default {
 
     if (!diffFile) return;
 
-    if (diffFile.highlighted_diff_lines) {
+    if (diffFile.highlighted_diff_lines.length) {
       diffFile.highlighted_diff_lines.find(l => l.line_code === lineCode).hasForm = hasForm;
     }
 
-    if (diffFile.parallel_diff_lines) {
+    if (diffFile.parallel_diff_lines.length) {
       const line = diffFile.parallel_diff_lines.find(l => {
         const { left, right } = l;
 
@@ -153,13 +155,13 @@ export default {
   },
 
   [types.EXPAND_ALL_FILES](state) {
-    state.diffFiles = state.diffFiles.map(file => ({
-      ...file,
-      viewer: {
-        ...file.viewer,
+    state.diffFiles.forEach(file => {
+      Object.assign(file, {
+        viewer: Object.assign(file.viewer, {
           collapsed: false,
-      },
-    }));
+        }),
+      });
+    });
   },
 
   [types.SET_LINE_DISCUSSIONS_FOR_FILE](state, { discussion, diffPositionByLineCode, hash }) {
@@ -197,29 +199,29 @@ export default {
       };
     };
 
-    state.diffFiles = state.diffFiles.map(diffFile => {
-      if (diffFile.file_hash === fileHash) {
-        const file = { ...diffFile };
-
-        if (file.highlighted_diff_lines) {
-          file.highlighted_diff_lines = file.highlighted_diff_lines.map(line =>
+    state.diffFiles.forEach(file => {
+      if (file.file_hash === fileHash) {
+        if (file.highlighted_diff_lines.length) {
+          file.highlighted_diff_lines.forEach(line => {
+            Object.assign(
+              line,
               setDiscussionsExpanded(lineCheck(line) ? mapDiscussions(line) : line),
             );
+          });
         }
 
-        if (file.parallel_diff_lines) {
-          file.parallel_diff_lines = file.parallel_diff_lines.map(line => {
+        if (file.parallel_diff_lines.length) {
+          file.parallel_diff_lines.forEach(line => {
             const left = line.left && lineCheck(line.left);
             const right = line.right && lineCheck(line.right);
 
             if (left || right) {
-              return {
-                ...line,
+              Object.assign(line, {
                 left: line.left ? setDiscussionsExpanded(mapDiscussions(line.left)) : null,
                 right: line.right
                   ? setDiscussionsExpanded(mapDiscussions(line.right, () => !left))
                   : null,
-              };
+              });
             }
 
             return line;
@@ -227,15 +229,15 @@ export default {
         }
 
         if (!file.parallel_diff_lines || !file.highlighted_diff_lines) {
-          file.discussions = (file.discussions || [])
+          const newDiscussions = (file.discussions || [])
             .filter(d => d.id !== discussion.id)
             .concat(discussion);
-        }
 
-        return file;
+          Object.assign(file, {
+            discussions: newDiscussions,
+          });
+        }
       }
-
-      return diffFile;
     });
   },
 
@@ -259,9 +261,9 @@ export default {
   [types.TOGGLE_LINE_DISCUSSIONS](state, { fileHash, lineCode, expanded }) {
     const selectedFile = state.diffFiles.find(f => f.file_hash === fileHash);
 
-    updateLineInFile(selectedFile, lineCode, line =>
-      Object.assign(line, { discussionsExpanded: expanded }),
-    );
+    updateLineInFile(selectedFile, lineCode, line => {
+      Object.assign(line, { discussionsExpanded: expanded });
+    });
   },
 
   [types.TOGGLE_FOLDER_OPEN](state, path) {

app/assets/javascripts/diffs/store/utils.js

@@ -185,6 +185,7 @@ export function addContextLines(options) {
  * Trims the first char of the `richText` property when it's either a space or a diff symbol.
  * @param {Object} line
  * @returns {Object}
+ * @deprecated
  */
 export function trimFirstCharOfLineContent(line = {}) {
   // eslint-disable-next-line no-param-reassign
@@ -212,80 +213,172 @@ function getLineCode({ left, right }, index) {
   return index;
 }
 
-// This prepares and optimizes the incoming diff data from the server
-// by setting up incremental rendering and removing unneeded data
-export function prepareDiffData(diffData) {
-  const filesLength = diffData.diff_files.length;
-  let showingLines = 0;
-  for (let i = 0; i < filesLength; i += 1) {
-    const file = diffData.diff_files[i];
+function diffFileUniqueId(file) {
+  return `${file.content_sha}-${file.file_hash}`;
+}
 
-    if (file.parallel_diff_lines) {
-      const linesLength = file.parallel_diff_lines.length;
-      for (let u = 0; u < linesLength; u += 1) {
-        const line = file.parallel_diff_lines[u];
+function combineDiffFilesWithPriorFiles(files, prior = []) {
+  files.forEach(file => {
+    const id = diffFileUniqueId(file);
+    const oldMatch = prior.find(oldFile => diffFileUniqueId(oldFile) === id);
 
-        line.line_code = getLineCode(line, u);
-        if (line.left) {
-          line.left = trimFirstCharOfLineContent(line.left);
-          line.left.discussions = [];
-          line.left.hasForm = false;
-        }
-        if (line.right) {
-          line.right = trimFirstCharOfLineContent(line.right);
-          line.right.discussions = [];
-          line.right.hasForm = false;
+    if (oldMatch) {
+      const missingInline = !file.highlighted_diff_lines;
+      const missingParallel = !file.parallel_diff_lines;
+
+      if (missingInline) {
+        Object.assign(file, {
+          highlighted_diff_lines: oldMatch.highlighted_diff_lines,
+        });
       }
+
+      if (missingParallel) {
+        Object.assign(file, {
+          parallel_diff_lines: oldMatch.parallel_diff_lines,
+        });
       }
     }
+  });
 
-    if (file.highlighted_diff_lines) {
-      const linesLength = file.highlighted_diff_lines.length;
-      for (let u = 0; u < linesLength; u += 1) {
-        const line = file.highlighted_diff_lines[u];
-        Object.assign(line, {
-          ...trimFirstCharOfLineContent(line),
+  return files;
+}
+
+function ensureBasicDiffFileLines(file) {
+  const missingInline = !file.highlighted_diff_lines;
+  const missingParallel = !file.parallel_diff_lines;
+
+  Object.assign(file, {
+    highlighted_diff_lines: missingInline ? [] : file.highlighted_diff_lines,
+    parallel_diff_lines: missingParallel ? [] : file.parallel_diff_lines,
+  });
+
+  return file;
+}
+
+function cleanRichText(text) {
+  return text ? text.replace(/^[+ -]/, '') : undefined;
+}
+
+function prepareLine(line) {
+  return Object.assign(line, {
+    rich_text: cleanRichText(line.rich_text),
+    discussionsExpanded: true,
     discussions: [],
     hasForm: false,
+    text: undefined,
   });
+}
+
+function prepareDiffFileLines(file) {
+  const inlineLines = file.highlighted_diff_lines;
+  const parallelLines = file.parallel_diff_lines;
+  let parallelLinesCount = 0;
+
+  inlineLines.forEach(prepareLine);
+
+  parallelLines.forEach((line, index) => {
+    Object.assign(line, { line_code: getLineCode(line, index) });
+
+    if (line.left) {
+      parallelLinesCount += 1;
+      prepareLine(line.left);
     }
-      showingLines += file.parallel_diff_lines.length;
+
+    if (line.right) {
+      parallelLinesCount += 1;
+      prepareLine(line.right);
     }
 
+    Object.assign(file, {
+      inlineLinesCount: inlineLines.length,
+      parallelLinesCount,
+    });
+  });
+
+  return file;
+}
+
+function getVisibleDiffLines(file) {
+  return Math.max(file.inlineLinesCount, file.parallelLinesCount);
+}
+
+function finalizeDiffFile(file) {
   const name = (file.viewer && file.viewer.name) || diffViewerModes.text;
+  const lines = getVisibleDiffLines(file);
 
   Object.assign(file, {
-      renderIt: showingLines < LINES_TO_BE_RENDERED_DIRECTLY,
-      collapsed: name === diffViewerModes.text && showingLines > MAX_LINES_TO_BE_RENDERED,
+    renderIt: lines < LINES_TO_BE_RENDERED_DIRECTLY,
+    collapsed: name === diffViewerModes.text && lines > MAX_LINES_TO_BE_RENDERED,
     isShowingFullFile: false,
     isLoadingFullFile: false,
     discussions: [],
     renderingLines: false,
   });
-  }
+
+  return file;
 }
 
-export function getDiffPositionByLineCode(diffFiles) {
-  return diffFiles.reduce((acc, diffFile) => {
-    // We can only use highlightedDiffLines to create the map of diff lines because
-    // highlightedDiffLines will also include every parallel diff line in it.
-    if (diffFile.highlighted_diff_lines) {
+export function prepareDiffData(diffData, priorFiles) {
+  return combineDiffFilesWithPriorFiles(diffData.diff_files, priorFiles)
+    .map(ensureBasicDiffFileLines)
+    .map(prepareDiffFileLines)
+    .map(finalizeDiffFile);
+}
+
+export function getDiffPositionByLineCode(diffFiles, useSingleDiffStyle) {
+  let lines = [];
+  const hasInlineDiffs = diffFiles.some(file => file.highlighted_diff_lines.length > 0);
+
+  if (!useSingleDiffStyle || hasInlineDiffs) {
+    // In either of these cases, we can use `highlighted_diff_lines` because
+    // that will include all of the parallel diff lines, too
+
+    lines = diffFiles.reduce((acc, diffFile) => {
       diffFile.highlighted_diff_lines.forEach(line => {
+        acc.push({ file: diffFile, line });
+      });
+
+      return acc;
+    }, []);
+  } else {
+    // If we're in single diff view mode and the inline lines haven't been
+    // loaded yet, we need to parse the parallel lines
+
+    lines = diffFiles.reduce((acc, diffFile) => {
+      diffFile.parallel_diff_lines.forEach(pair => {
+        // It's possible for a parallel line to have an opposite line that doesn't exist
+        // For example: *deleted* lines will have `null` right lines, while
+        // *added* lines will have `null` left lines.
+        // So we have to check each line before we push it onto the array so we're not
+        // pushing null line diffs
+
+        if (pair.left) {
+          acc.push({ file: diffFile, line: pair.left });
+        }
+
+        if (pair.right) {
+          acc.push({ file: diffFile, line: pair.right });
+        }
+      });
+
+      return acc;
+    }, []);
+  }
+
+  return lines.reduce((acc, { file, line }) => {
     if (line.line_code) {
       acc[line.line_code] = {
-            base_sha: diffFile.diff_refs.base_sha,
-            head_sha: diffFile.diff_refs.head_sha,
-            start_sha: diffFile.diff_refs.start_sha,
-            new_path: diffFile.new_path,
-            old_path: diffFile.old_path,
+        base_sha: file.diff_refs.base_sha,
+        head_sha: file.diff_refs.head_sha,
+        start_sha: file.diff_refs.start_sha,
+        new_path: file.new_path,
+        old_path: file.old_path,
         old_line: line.old_line,
         new_line: line.new_line,
         line_code: line.line_code,
         position_type: 'text',
       };
     }
-      });
-    }
 
     return acc;
   }, {});
@@ -462,47 +555,47 @@ export const convertExpandLines = ({
 
 export const idleCallback = cb => requestIdleCallback(cb);
 
-export const updateLineInFile = (selectedFile, lineCode, updateFn) => {
-  if (selectedFile.parallel_diff_lines) {
-    const targetLine = selectedFile.parallel_diff_lines.find(
-      line =>
-        (line.left && line.left.line_code === lineCode) ||
-        (line.right && line.right.line_code === lineCode),
-    );
-    if (targetLine) {
-      const side = targetLine.left && targetLine.left.line_code === lineCode ? 'left' : 'right';
+function getLinesFromFileByLineCode(file, lineCode) {
+  const parallelLines = file.parallel_diff_lines;
+  const inlineLines = file.highlighted_diff_lines;
+  const matchesCode = line => line.line_code === lineCode;
 
-      updateFn(targetLine[side]);
-    }
+  return [
+    ...parallelLines.reduce((acc, line) => {
+      if (line.left) {
+        acc.push(line.left);
       }
-  if (selectedFile.highlighted_diff_lines) {
-    const targetInlineLine = selectedFile.highlighted_diff_lines.find(
-      line => line.line_code === lineCode,
-    );
 
-    if (targetInlineLine) {
-      updateFn(targetInlineLine);
-    }
+      if (line.right) {
+        acc.push(line.right);
       }
+
+      return acc;
+    }, []),
+    ...inlineLines,
+  ].filter(matchesCode);
+}
+
+export const updateLineInFile = (selectedFile, lineCode, updateFn) => {
+  getLinesFromFileByLineCode(selectedFile, lineCode).forEach(updateFn);
 };
 
 export const allDiscussionWrappersExpanded = diff => {
-  const discussionsExpandedArray = [];
-  if (diff.parallel_diff_lines) {
-    diff.parallel_diff_lines.forEach(line => {
-      if (line.left && line.left.discussions.length) {
-        discussionsExpandedArray.push(line.left.discussionsExpanded);
-      }
-      if (line.right && line.right.discussions.length) {
-        discussionsExpandedArray.push(line.right.discussionsExpanded);
+  let discussionsExpanded = true;
+  const changeExpandedResult = line => {
+    if (line && line.discussions.length) {
+      discussionsExpanded = discussionsExpanded && line.discussionsExpanded;
     }
+  };
+
+  diff.parallel_diff_lines.forEach(line => {
+    changeExpandedResult(line.left);
+    changeExpandedResult(line.right);
   });
-  } else if (diff.highlighted_diff_lines) {
+
   diff.highlighted_diff_lines.forEach(line => {
-      if (line.discussions.length) {
-        discussionsExpandedArray.push(line.discussionsExpanded);
-      }
+    changeExpandedResult(line);
   });
-  }
-  return discussionsExpandedArray.every(el => el);
+
+  return discussionsExpanded;
 };

app/assets/javascripts/helpers/diffs_helper.js

+export function hasInlineLines(diffFile) {
+  return diffFile?.highlighted_diff_lines?.length > 0; /* eslint-disable-line camelcase */
+}
+
+export function hasParallelLines(diffFile) {
+  return diffFile?.parallel_diff_lines?.length > 0; /* eslint-disable-line camelcase */
+}
+
+export function isSingleViewStyle(diffFile) {
+  return !hasParallelLines(diffFile) || !hasInlineLines(diffFile);
+}
+
+export function hasDiff(diffFile) {
+  return (
+    hasInlineLines(diffFile) ||
+    hasParallelLines(diffFile) ||
+    !diffFile?.blob?.readable_text /* eslint-disable-line camelcase */
+  );
+}

changelogs/unreleased/feature-split-diff-attempt-3.yml

+---
+title: Load MR diff types lazily to reduce initial diff payload size
+merge_request: 19930
+author:
+type: added

ee/spec/features/merge_request/batch_comments_spec.rb

@@ -31,11 +31,8 @@ describe 'Merge request > Batch comments', :js do
     end
   end
 
-  it_behaves_like 'rendering a single diff version'
-
   context 'Feature is enabled' do
     before do
-      stub_feature_flags(single_mr_diff_view: false)
       stub_feature_flags(diffs_batch_load: false)
       stub_licensed_features(batch_comments: true)
 

ee/spec/features/merge_request/user_reviews_image_spec.rb

@@ -11,7 +11,6 @@ describe 'Merge request > image review', :js do
   let(:merge_request) { create(:merge_request_with_diffs, :with_image_diffs, source_project: project, author: user) }
 
   before do
-    stub_feature_flags(single_mr_diff_view: false)
     stub_licensed_features(batch_comments: true)
 
     sign_in(user)
@@ -24,8 +23,6 @@ describe 'Merge request > image review', :js do
     wait_for_requests
   end
 
-  it_behaves_like 'rendering a single diff version'
-
   it 'leaves review' do
     find('.js-add-image-diff-note-button', match: :first).click
 

spec/features/merge_request/maintainer_edits_fork_spec.rb

@@ -32,8 +32,6 @@ describe 'a maintainer edits files on a source-branch of an MR from a fork', :js
     wait_for_requests
   end
 
-  it_behaves_like 'rendering a single diff version'
-
   it 'mentions commits will go to the source branch' do
     expect(page).to have_content('Your changes can be committed to fix because a merge request is open.')
   end

spec/features/merge_request/user_comments_on_diff_spec.rb

@@ -13,15 +13,12 @@ describe 'User comments on a diff', :js do
   let(:user) { create(:user) }
 
   before do
-    stub_feature_flags(single_mr_diff_view: false)
     project.add_maintainer(user)
     sign_in(user)
 
     visit(diffs_project_merge_request_path(project, merge_request))
   end
 
-  it_behaves_like 'rendering a single diff version'
-
   context 'when viewing comments' do
     context 'when toggling inline comments' do
       context 'in a single file' do

spec/features/merge_request/user_creates_image_diff_notes_spec.rb

@@ -9,7 +9,6 @@ describe 'Merge request > User creates image diff notes', :js do
   let(:user) { project.creator }
 
   before do
-    stub_feature_flags(single_mr_diff_view: false)
     sign_in(user)
 
     # Stub helper to return any blob file as image from public app folder.
@@ -18,8 +17,6 @@ describe 'Merge request > User creates image diff notes', :js do
     allow_any_instance_of(DiffHelper).to receive(:diff_file_old_blob_raw_url).and_return('/favicon.png')
   end
 
-  it_behaves_like 'rendering a single diff version'
-
   context 'create commit diff notes' do
     commit_id = '2f63565e7aac07bcdadb654e253078b727143ec4'
 

spec/features/merge_request/user_expands_diff_spec.rb

@@ -7,7 +7,6 @@ describe 'User expands diff', :js do
   let(:merge_request) { create(:merge_request, source_branch: 'expand-collapse-files', source_project: project, target_project: project) }
 
   before do
-    stub_feature_flags(single_mr_diff_view: false)
     stub_feature_flags(diffs_batch_load: false)
 
     allow(Gitlab::Git::Diff).to receive(:size_limit).and_return(100.kilobytes)
@@ -18,8 +17,6 @@ describe 'User expands diff', :js do
     wait_for_requests
   end
 
-  it_behaves_like 'rendering a single diff version'
-
   it 'allows user to expand diff' do
     page.within find('[id="6eb14e00385d2fb284765eb1cd8d420d33d63fc9"]') do
       click_link 'Click to expand it.'

spec/features/merge_request/user_posts_diff_notes_spec.rb

@@ -14,15 +14,12 @@ describe 'Merge request > User posts diff notes', :js do
   let(:test_note_comment) { 'this is a test note!' }
 
   before do
-    stub_feature_flags(single_mr_diff_view: false)
     set_cookie('sidebar_collapsed', 'true')
 
     project.add_developer(user)
     sign_in(user)
   end
 
-  it_behaves_like 'rendering a single diff version'
-
   context 'when hovering over a parallel view diff file' do
     before do
       visit diffs_project_merge_request_path(project, merge_request, view: 'parallel')

spec/features/merge_request/user_resolves_conflicts_spec.rb

@@ -9,7 +9,6 @@ describe 'Merge request > User resolves conflicts', :js do
   before do
     # In order to have the diffs collapsed, we need to disable the increase feature
     stub_feature_flags(gitlab_git_diff_size_limit_increase: false)
-    stub_feature_flags(single_mr_diff_view: false)
   end
 
   def create_merge_request(source_branch)
@@ -18,8 +17,6 @@ describe 'Merge request > User resolves conflicts', :js do
     end
   end
 
-  it_behaves_like 'rendering a single diff version'
-
   shared_examples 'conflicts are resolved in Interactive mode' do
     it 'conflicts are resolved in Interactive mode' do
       within find('.files-wrapper .diff-file', text: 'files/ruby/popen.rb') do

spec/features/merge_request/user_resolves_diff_notes_and_discussions_resolve_spec.rb

@@ -20,12 +20,9 @@ describe 'Merge request > User resolves diff notes and threads', :js do
   end
 
   before do
-    stub_feature_flags(single_mr_diff_view: false)
     stub_feature_flags(diffs_batch_load: false)
   end
 
-  it_behaves_like 'rendering a single diff version'
-
   context 'no threads' do
     before do
       project.add_maintainer(user)

spec/features/merge_request/user_sees_avatar_on_diff_notes_spec.rb

@@ -21,7 +21,6 @@ describe 'Merge request > User sees avatars on diff notes', :js do
   let!(:note) { create(:diff_note_on_merge_request, project: project, noteable: merge_request, position: position) }
 
   before do
-    stub_feature_flags(single_mr_diff_view: false)
     stub_feature_flags(diffs_batch_load: false)
     project.add_maintainer(user)
     sign_in user
@@ -29,8 +28,6 @@ describe 'Merge request > User sees avatars on diff notes', :js do
     set_cookie('sidebar_collapsed', 'true')
   end
 
-  it_behaves_like 'rendering a single diff version'
-
   context 'discussion tab' do
     before do
       visit project_merge_request_path(project, merge_request)

spec/features/merge_request/user_sees_diff_spec.rb

@@ -10,12 +10,9 @@ describe 'Merge request > User sees diff', :js do
   let(:merge_request) { create(:merge_request, source_project: project) }
 
   before do
-    stub_feature_flags(single_mr_diff_view: false)
     stub_feature_flags(diffs_batch_load: false)
   end
 
-  it_behaves_like 'rendering a single diff version'
-
   context 'when linking to note' do
     describe 'with unresolved note' do
       let(:note) { create :diff_note_on_merge_request, project: project, noteable: merge_request }

spec/features/merge_request/user_sees_mr_with_deleted_source_branch_spec.rb

@@ -11,14 +11,11 @@ describe 'Merge request > User sees MR with deleted source branch', :js do
   let(:user) { project.creator }
 
   before do
-    stub_feature_flags(single_mr_diff_view: false)
     merge_request.update!(source_branch: 'this-branch-does-not-exist')
     sign_in(user)
     visit project_merge_request_path(project, merge_request)
   end
 
-  it_behaves_like 'rendering a single diff version'
-
   it 'shows a message about missing source branch' do
     expect(page).to have_content('Source branch does not exist.')
   end

spec/features/merge_request/user_sees_versions_spec.rb

@@ -16,7 +16,6 @@ describe 'Merge request > User sees versions', :js do
   let!(:params) { {} }
 
   before do
-    stub_feature_flags(single_mr_diff_view: false)
     stub_feature_flags(diffs_batch_load: false)
 
     project.add_maintainer(user)
@@ -24,8 +23,6 @@ describe 'Merge request > User sees versions', :js do
     visit diffs_project_merge_request_path(project, merge_request, params)
   end
 
-  it_behaves_like 'rendering a single diff version'
-
   shared_examples 'allows commenting' do |file_id:, line_code:, comment:|
     it do
       diff_file_selector = ".diff-file[id='#{file_id}']"

spec/features/merge_request/user_suggests_changes_on_diff_spec.rb

@@ -25,15 +25,12 @@ describe 'User comments on a diff', :js do
   let(:user) { create(:user) }
 
   before do
-    stub_feature_flags(single_mr_diff_view: false)
     project.add_maintainer(user)
     sign_in(user)
 
     visit(diffs_project_merge_request_path(project, merge_request))
   end
 
-  it_behaves_like 'rendering a single diff version'
-
   context 'single suggestion note' do
     it 'hides suggestion popover' do
       click_diff_line(find("[id='#{sample_compare.changes[1][:line_code]}']"))

spec/features/merge_request/user_toggles_whitespace_changes_spec.rb

@@ -8,7 +8,6 @@ describe 'Merge request > User toggles whitespace changes', :js do
   let(:user) { project.creator }
 
   before do
-    stub_feature_flags(single_mr_diff_view: false)
     project.add_maintainer(user)
     sign_in(user)
     visit diffs_project_merge_request_path(project, merge_request)
@@ -16,8 +15,6 @@ describe 'Merge request > User toggles whitespace changes', :js do
     find('.js-show-diff-settings').click
   end
 
-  it_behaves_like 'rendering a single diff version'
-
   it 'has a button to toggle whitespace changes' do
     expect(page).to have_content 'Show whitespace changes'
   end

spec/features/merge_request/user_views_diffs_spec.rb

@@ -9,7 +9,6 @@ describe 'User views diffs', :js do
   let(:project) { create(:project, :public, :repository) }
 
   before do
-    stub_feature_flags(single_mr_diff_view: false)
     stub_feature_flags(diffs_batch_load: false)
     visit(diffs_project_merge_request_path(project, merge_request))
 
@@ -18,8 +17,6 @@ describe 'User views diffs', :js do
     find('.js-toggle-tree-list').click
   end
 
-  it_behaves_like 'rendering a single diff version'
-
   shared_examples 'unfold diffs' do
     it 'unfolds diffs upwards' do
       first('.js-unfold').click

spec/features/projects/blobs/edit_spec.rb

@@ -12,11 +12,9 @@ describe 'Editing file blob', :js do
   let(:readme_file_path) { 'README.md' }
 
   before do
-    stub_feature_flags(web_ide_default: false, single_mr_diff_view: false)
+    stub_feature_flags(web_ide_default: false)
   end
 
-  it_behaves_like 'rendering a single diff version'
-
   context 'as a developer' do
     let(:user) { create(:user) }
     let(:role) { :developer }

spec/features/projects/view_on_env_spec.rb

@@ -9,14 +9,11 @@ describe 'View on environment', :js do
   let(:user) { project.creator }
 
   before do
-    stub_feature_flags(single_mr_diff_view: false)
     stub_feature_flags(diffs_batch_load: false)
 
     project.add_maintainer(user)
   end
 
-  it_behaves_like 'rendering a single diff version'
-
   context 'when the branch has a route map' do
     let(:route_map) do
       <<-MAP.strip_heredoc

spec/frontend/helpers/diffs_helper_spec.js

+import * as diffsHelper from '~/helpers/diffs_helper';
+
+describe('diffs helper', () => {
+  function getDiffFile(withOverrides = {}) {
+    return {
+      parallel_diff_lines: ['line'],
+      highlighted_diff_lines: ['line'],
+      blob: {
+        readable_text: 'text',
+      },
+      ...withOverrides,
+    };
+  }
+
+  describe('hasInlineLines', () => {
+    it('is false when the file does not exist', () => {
+      expect(diffsHelper.hasInlineLines()).toBeFalsy();
+    });
+
+    it('is false when the file does not have the highlighted_diff_lines property', () => {
+      const missingInline = getDiffFile({ highlighted_diff_lines: undefined });
+
+      expect(diffsHelper.hasInlineLines(missingInline)).toBeFalsy();
+    });
+
+    it('is false when the file has zero highlighted_diff_lines', () => {
+      const emptyInline = getDiffFile({ highlighted_diff_lines: [] });
+
+      expect(diffsHelper.hasInlineLines(emptyInline)).toBeFalsy();
+    });
+
+    it('is true when the file has at least 1 highlighted_diff_lines', () => {
+      expect(diffsHelper.hasInlineLines(getDiffFile())).toBeTruthy();
+    });
+  });
+
+  describe('hasParallelLines', () => {
+    it('is false when the file does not exist', () => {
+      expect(diffsHelper.hasParallelLines()).toBeFalsy();
+    });
+
+    it('is false when the file does not have the parallel_diff_lines property', () => {
+      const missingInline = getDiffFile({ parallel_diff_lines: undefined });
+
+      expect(diffsHelper.hasParallelLines(missingInline)).toBeFalsy();
+    });
+
+    it('is false when the file has zero parallel_diff_lines', () => {
+      const emptyInline = getDiffFile({ parallel_diff_lines: [] });
+
+      expect(diffsHelper.hasParallelLines(emptyInline)).toBeFalsy();
+    });
+
+    it('is true when the file has at least 1 parallel_diff_lines', () => {
+      expect(diffsHelper.hasParallelLines(getDiffFile())).toBeTruthy();
+    });
+  });
+
+  describe('isSingleViewStyle', () => {
+    it('is true when the file has at least 1 inline line but no parallel lines for any reason', () => {
+      const noParallelLines = getDiffFile({ parallel_diff_lines: undefined });
+      const emptyParallelLines = getDiffFile({ parallel_diff_lines: [] });
+
+      expect(diffsHelper.isSingleViewStyle(noParallelLines)).toBeTruthy();
+      expect(diffsHelper.isSingleViewStyle(emptyParallelLines)).toBeTruthy();
+    });
+
+    it('is true when the file has at least 1 parallel line but no inline lines for any reason', () => {
+      const noInlineLines = getDiffFile({ highlighted_diff_lines: undefined });
+      const emptyInlineLines = getDiffFile({ highlighted_diff_lines: [] });
+
+      expect(diffsHelper.isSingleViewStyle(noInlineLines)).toBeTruthy();
+      expect(diffsHelper.isSingleViewStyle(emptyInlineLines)).toBeTruthy();
+    });
+
+    it('is true when the file does not have any inline lines or parallel lines for any reason', () => {
+      const noLines = getDiffFile({
+        highlighted_diff_lines: undefined,
+        parallel_diff_lines: undefined,
+      });
+      const emptyLines = getDiffFile({
+        highlighted_diff_lines: [],
+        parallel_diff_lines: [],
+      });
+
+      expect(diffsHelper.isSingleViewStyle(noLines)).toBeTruthy();
+      expect(diffsHelper.isSingleViewStyle(emptyLines)).toBeTruthy();
+      expect(diffsHelper.isSingleViewStyle()).toBeTruthy();
+    });
+
+    it('is false when the file has both inline and parallel lines', () => {
+      expect(diffsHelper.isSingleViewStyle(getDiffFile())).toBeFalsy();
+    });
+  });
+
+  describe.each`
+    context                                     | inline       | parallel     | blob                         | expected
+    ${'only has inline lines'}                  | ${['line']}  | ${undefined} | ${undefined}                 | ${true}
+    ${'only has parallel lines'}                | ${undefined} | ${['line']}  | ${undefined}                 | ${true}
+    ${"doesn't have inline, parallel, or blob"} | ${undefined} | ${undefined} | ${undefined}                 | ${true}
+    ${'has blob readable text'}                 | ${undefined} | ${undefined} | ${{ readable_text: 'text' }} | ${false}
+  `('when hasDiff', ({ context, inline, parallel, blob, expected }) => {
+    it(`${context}`, () => {
+      const diffFile = getDiffFile({
+        highlighted_diff_lines: inline,
+        parallel_diff_lines: parallel,
+        blob,
+      });
+
+      expect(diffsHelper.hasDiff(diffFile)).toEqual(expected);
+    });
+  });
+});

spec/javascripts/diffs/components/app_spec.js

@@ -10,6 +10,7 @@ import CompareVersions from '~/diffs/components/compare_versions.vue';
 import HiddenFilesWarning from '~/diffs/components/hidden_files_warning.vue';
 import CommitWidget from '~/diffs/components/commit_widget.vue';
 import TreeList from '~/diffs/components/tree_list.vue';
+import { INLINE_DIFF_VIEW_TYPE, PARALLEL_DIFF_VIEW_TYPE } from '~/diffs/constants';
 import createDiffsStore from '../create_diffs_store';
 import diffsMockData from '../mock_data/merge_request_diffs';
 
@@ -41,7 +42,6 @@ describe('diffs/components/app', () => {
         changesEmptyStateIllustration: '',
         dismissEndpoint: '',
         showSuggestPopover: true,
-        useSingleDiffStyle: false,
         ...props,
       },
       store,
@@ -53,6 +53,12 @@ describe('diffs/components/app', () => {
     });
   }
 
+  function getOppositeViewType(currentViewType) {
+    return currentViewType === INLINE_DIFF_VIEW_TYPE
+      ? PARALLEL_DIFF_VIEW_TYPE
+      : INLINE_DIFF_VIEW_TYPE;
+  }
+
   beforeEach(() => {
     // setup globals (needed for component to mount :/)
     window.mrTabs = jasmine.createSpyObj('mrTabs', ['resetViewContainer']);
@@ -82,9 +88,146 @@ describe('diffs/components/app', () => {
       spyOn(wrapper.vm, 'startRenderDiffsQueue');
       spyOn(wrapper.vm, 'unwatchDiscussions');
       store.state.diffs.retrievingBatches = true;
+      store.state.diffs.diffFiles = [];
       wrapper.vm.$nextTick(done);
     });
 
+    describe('when the diff view type changes and it should load a single diff view style', () => {
+      const noLinesDiff = {
+        highlighted_diff_lines: [],
+        parallel_diff_lines: [],
+      };
+      const parallelLinesDiff = {
+        highlighted_diff_lines: [],
+        parallel_diff_lines: ['line'],
+      };
+      const inlineLinesDiff = {
+        highlighted_diff_lines: ['line'],
+        parallel_diff_lines: [],
+      };
+      const fullDiff = {
+        highlighted_diff_lines: ['line'],
+        parallel_diff_lines: ['line'],
+      };
+
+      function expectFetchToOccur({
+        vueInstance,
+        done = () => {},
+        batch = false,
+        existingFiles = 1,
+      } = {}) {
+        vueInstance.$nextTick(() => {
+          expect(vueInstance.diffFiles.length).toEqual(existingFiles);
+
+          if (!batch) {
+            expect(vueInstance.fetchDiffFiles).toHaveBeenCalled();
+            expect(vueInstance.fetchDiffFilesBatch).not.toHaveBeenCalled();
+          } else {
+            expect(vueInstance.fetchDiffFiles).not.toHaveBeenCalled();
+            expect(vueInstance.fetchDiffFilesBatch).toHaveBeenCalled();
+          }
+
+          done();
+        });
+      }
+
+      beforeEach(() => {
+        wrapper.vm.glFeatures.singleMrDiffView = true;
+      });
+
+      it('fetches diffs if it has none', done => {
+        wrapper.vm.isLatestVersion = () => false;
+
+        store.state.diffs.diffViewType = getOppositeViewType(wrapper.vm.diffViewType);
+
+        expectFetchToOccur({ vueInstance: wrapper.vm, batch: false, existingFiles: 0, done });
+      });
+
+      it('fetches diffs if it has both view styles, but no lines in either', done => {
+        wrapper.vm.isLatestVersion = () => false;
+
+        store.state.diffs.diffFiles.push(noLinesDiff);
+        store.state.diffs.diffViewType = getOppositeViewType(wrapper.vm.diffViewType);
+
+        expectFetchToOccur({ vueInstance: wrapper.vm, done });
+      });
+
+      it('fetches diffs if it only has inline view style', done => {
+        wrapper.vm.isLatestVersion = () => false;
+
+        store.state.diffs.diffFiles.push(inlineLinesDiff);
+        store.state.diffs.diffViewType = getOppositeViewType(wrapper.vm.diffViewType);
+
+        expectFetchToOccur({ vueInstance: wrapper.vm, done });
+      });
+
+      it('fetches diffs if it only has parallel view style', done => {
+        wrapper.vm.isLatestVersion = () => false;
+
+        store.state.diffs.diffFiles.push(parallelLinesDiff);
+        store.state.diffs.diffViewType = getOppositeViewType(wrapper.vm.diffViewType);
+
+        expectFetchToOccur({ vueInstance: wrapper.vm, done });
+      });
+
+      it('fetches batch diffs if it has none', done => {
+        wrapper.vm.glFeatures.diffsBatchLoad = true;
+
+        store.state.diffs.diffViewType = getOppositeViewType(wrapper.vm.diffViewType);
+
+        expectFetchToOccur({ vueInstance: wrapper.vm, batch: true, existingFiles: 0, done });
+      });
+
+      it('fetches batch diffs if it has both view styles, but no lines in either', done => {
+        wrapper.vm.glFeatures.diffsBatchLoad = true;
+
+        store.state.diffs.diffFiles.push(noLinesDiff);
+        store.state.diffs.diffViewType = getOppositeViewType(wrapper.vm.diffViewType);
+
+        expectFetchToOccur({ vueInstance: wrapper.vm, batch: true, done });
+      });
+
+      it('fetches batch diffs if it only has inline view style', done => {
+        wrapper.vm.glFeatures.diffsBatchLoad = true;
+
+        store.state.diffs.diffFiles.push(inlineLinesDiff);
+        store.state.diffs.diffViewType = getOppositeViewType(wrapper.vm.diffViewType);
+
+        expectFetchToOccur({ vueInstance: wrapper.vm, batch: true, done });
+      });
+
+      it('fetches batch diffs if it only has parallel view style', done => {
+        wrapper.vm.glFeatures.diffsBatchLoad = true;
+
+        store.state.diffs.diffFiles.push(parallelLinesDiff);
+        store.state.diffs.diffViewType = getOppositeViewType(wrapper.vm.diffViewType);
+
+        expectFetchToOccur({ vueInstance: wrapper.vm, batch: true, done });
+      });
+
+      it('does not fetch diffs if it has already fetched both styles of diff', () => {
+        wrapper.vm.glFeatures.diffsBatchLoad = false;
+
+        store.state.diffs.diffFiles.push(fullDiff);
+        store.state.diffs.diffViewType = getOppositeViewType(wrapper.vm.diffViewType);
+
+        expect(wrapper.vm.diffFiles.length).toEqual(1);
+        expect(wrapper.vm.fetchDiffFiles).not.toHaveBeenCalled();
+        expect(wrapper.vm.fetchDiffFilesBatch).not.toHaveBeenCalled();
+      });
+
+      it('does not fetch batch diffs if it has already fetched both styles of diff', () => {
+        wrapper.vm.glFeatures.diffsBatchLoad = true;
+
+        store.state.diffs.diffFiles.push(fullDiff);
+        store.state.diffs.diffViewType = getOppositeViewType(wrapper.vm.diffViewType);
+
+        expect(wrapper.vm.diffFiles.length).toEqual(1);
+        expect(wrapper.vm.fetchDiffFiles).not.toHaveBeenCalled();
+        expect(wrapper.vm.fetchDiffFilesBatch).not.toHaveBeenCalled();
+      });
+    });
+
     it('calls fetchDiffFiles if diffsBatchLoad is not enabled', done => {
       wrapper.vm.glFeatures.diffsBatchLoad = false;
       wrapper.vm.fetchData(false);

spec/javascripts/diffs/store/actions_spec.js

@@ -120,7 +120,7 @@ describe('DiffsStoreActions', () => {
 
   describe('fetchDiffFiles', () => {
     it('should fetch diff files', done => {
-      const endpoint = '/fetch/diff/files?w=1';
+      const endpoint = '/fetch/diff/files?view=inline&w=1';
       const mock = new MockAdapter(axios);
       const res = { diff_files: 1, merge_request_diffs: [] };
       mock.onGet(endpoint).reply(200, res);
@@ -128,7 +128,7 @@ describe('DiffsStoreActions', () => {
       testAction(
         fetchDiffFiles,
         {},
-        { endpoint },
+        { endpoint, diffFiles: [], showWhitespace: false, diffViewType: 'inline' },
         [
           { type: types.SET_LOADING, payload: true },
           { type: types.SET_LOADING, payload: false },

spec/javascripts/diffs/store/mutations_spec.js

@@ -52,7 +52,14 @@ describe('DiffsStoreMutations', () => {
 
   describe('SET_DIFF_DATA', () => {
     it('should set diff data type properly', () => {
-      const state = {};
+      const state = {
+        diffFiles: [
+          {
+            content_sha: diffFileMockData.content_sha,
+            file_hash: diffFileMockData.file_hash,
+          },
+        ],
+      };
       const diffMock = {
         diff_files: [diffFileMockData],
       };
@@ -62,9 +69,41 @@ describe('DiffsStoreMutations', () => {
       const firstLine = state.diffFiles[0].parallel_diff_lines[0];
 
       expect(firstLine.right.text).toBeUndefined();
+      expect(state.diffFiles.length).toEqual(1);
       expect(state.diffFiles[0].renderIt).toEqual(true);
       expect(state.diffFiles[0].collapsed).toEqual(false);
     });
+
+    describe('given diffsBatchLoad feature flag is enabled', () => {
+      beforeEach(() => {
+        gon.features = { diffsBatchLoad: true };
+      });
+
+      afterEach(() => {
+        delete gon.features;
+      });
+
+      it('should not modify the existing state', () => {
+        const state = {
+          diffFiles: [
+            {
+              content_sha: diffFileMockData.content_sha,
+              file_hash: diffFileMockData.file_hash,
+              highlighted_diff_lines: [],
+            },
+          ],
+        };
+        const diffMock = {
+          diff_files: [diffFileMockData],
+        };
+
+        mutations[types.SET_DIFF_DATA](state, diffMock);
+
+        // If the batch load is enabled, there shouldn't be any processing
+        // done on the existing state object, so we shouldn't have this.
+        expect(state.diffFiles[0].parallel_diff_lines).toBeUndefined();
+      });
+    });
   });
 
   describe('SET_DIFFSET_DIFF_DATA_BATCH_DATA', () => {
@@ -168,11 +207,17 @@ describe('DiffsStoreMutations', () => {
     it('should update the state with the given data for the given file hash', () => {
       const fileHash = 123;
       const state = {
-        diffFiles: [{}, { file_hash: fileHash, existing_field: 0 }],
+        diffFiles: [{}, { content_sha: 'abc', file_hash: fileHash, existing_field: 0 }],
       };
       const data = {
         diff_files: [
-          { file_hash: fileHash, extra_field: 1, existing_field: 1, viewer: { name: 'text' } },
+          {
+            content_sha: 'abc',
+            file_hash: fileHash,
+            extra_field: 1,
+            existing_field: 1,
+            viewer: { name: 'text' },
+          },
         ],
       };
 
@@ -208,7 +253,7 @@ describe('DiffsStoreMutations', () => {
                   discussions: [],
                 },
                 right: {
-                  line_code: 'ABC_1',
+                  line_code: 'ABC_2',
                   discussions: [],
                 },
               },
@@ -274,7 +319,7 @@ describe('DiffsStoreMutations', () => {
                   discussions: [],
                 },
                 right: {
-                  line_code: 'ABC_1',
+                  line_code: 'ABC_2',
                   discussions: [],
                 },
               },
@@ -352,7 +397,7 @@ describe('DiffsStoreMutations', () => {
                   discussions: [],
                 },
                 right: {
-                  line_code: 'ABC_1',
+                  line_code: 'ABC_2',
                   discussions: [],
                 },
               },
@@ -448,6 +493,7 @@ describe('DiffsStoreMutations', () => {
                 discussions: [],
               },
             ],
+            parallel_diff_lines: [],
           },
         ],
       };

spec/javascripts/diffs/store/utils_spec.js

@@ -314,11 +314,29 @@ describe('DiffsStoreUtils', () => {
   });
 
   describe('prepareDiffData', () => {
+    let mock;
     let preparedDiff;
+    let splitInlineDiff;
+    let splitParallelDiff;
+    let completedDiff;
 
     beforeEach(() => {
-      preparedDiff = { diff_files: [getDiffFileMock()] };
+      mock = getDiffFileMock();
+      preparedDiff = { diff_files: [mock] };
+      splitInlineDiff = {
+        diff_files: [Object.assign({}, mock, { parallel_diff_lines: undefined })],
+      };
+      splitParallelDiff = {
+        diff_files: [Object.assign({}, mock, { highlighted_diff_lines: undefined })],
+      };
+      completedDiff = {
+        diff_files: [Object.assign({}, mock, { highlighted_diff_lines: undefined })],
+      };
+
       utils.prepareDiffData(preparedDiff);
+      utils.prepareDiffData(splitInlineDiff);
+      utils.prepareDiffData(splitParallelDiff);
+      utils.prepareDiffData(completedDiff, [mock]);
     });
 
     it('sets the renderIt and collapsed attribute on files', () => {
@@ -359,6 +377,19 @@ describe('DiffsStoreUtils', () => {
 
       expect(firstLine.line_code).toEqual(firstLine.right.line_code);
     });
+
+    it('guarantees an empty array for both diff styles', () => {
+      expect(splitInlineDiff.diff_files[0].parallel_diff_lines.length).toEqual(0);
+      expect(splitInlineDiff.diff_files[0].highlighted_diff_lines.length).toBeGreaterThan(0);
+      expect(splitParallelDiff.diff_files[0].parallel_diff_lines.length).toBeGreaterThan(0);
+      expect(splitParallelDiff.diff_files[0].highlighted_diff_lines.length).toEqual(0);
+    });
+
+    it('merges existing diff files with newly loaded diff files to ensure split diffs are eventually completed', () => {
+      expect(completedDiff.diff_files.length).toEqual(1);
+      expect(completedDiff.diff_files[0].parallel_diff_lines.length).toBeGreaterThan(0);
+      expect(completedDiff.diff_files[0].highlighted_diff_lines.length).toBeGreaterThan(0);
+    });
   });
 
   describe('isDiscussionApplicableToLine', () => {

spec/support/shared_examples/merge_requests_rendering_a_single_diff_version.rb

-# frozen_string_literal: true
-
-# This pending test can be removed when `single_mr_diff_view` is enabled by default
-# disabling the feature flag above is then not needed anymore.
-RSpec.shared_examples 'rendering a single diff version' do |attribute|
-  before do
-    stub_feature_flags(diffs_batch_load: false)
-  end
-
-  pending 'allows editing diff settings single_mr_diff_view is enabled' do
-    project = create(:project, :repository)
-    user = project.creator
-    merge_request = create(:merge_request, source_project: project)
-    stub_feature_flags(single_mr_diff_view: true)
-    sign_in(user)
-
-    visit(diffs_project_merge_request_path(project, merge_request))
-
-    expect(page).to have_selector('.js-show-diff-settings')
-  end
-end