Fixes Web IDE not loading merge request files

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/60243

app/assets/javascripts/ide/stores/mutations/merge_request.js

@@ -7,6 +7,8 @@ export default {
     });
   },
   [types.SET_MERGE_REQUEST](state, { projectPath, mergeRequestId, mergeRequest }) {
+    const existingMergeRequest = state.projects[projectPath].mergeRequests[mergeRequestId] || {};
+
     Object.assign(state.projects[projectPath], {
       mergeRequests: {
         [mergeRequestId]: {
@@ -15,6 +17,7 @@ export default {
           changes: [],
           versions: [],
           baseCommitSha: null,
+          ...existingMergeRequest,
         },
       },
     });

changelogs/unreleased/fixed-web-ide-merge-request-review.yml

+---
+title: Fixed Web IDE not loading merge request files
+merge_request:
+author:
+type: fixed

spec/features/ide/user_opens_merge_request_spec.rb

+require 'spec_helper'
+
+describe 'IDE merge request', :js do
+  let(:merge_request) { create(:merge_request, :with_diffs, :simple, source_project: project) }
+  let(:project) { create(:project, :public, :repository) }
+  let(:user) { project.owner }
+
+  before do
+    sign_in(user)
+
+    visit(merge_request_path(merge_request))
+  end
+
+  it 'user opens merge request' do
+    click_link 'Open in Web IDE'
+
+    wait_for_requests
+
+    expect(page).to have_selector('.monaco-diff-editor')
+  end
+end

spec/frontend/ide/stores/mutations/merge_request_spec.js

@@ -32,6 +32,24 @@ describe('IDE store merge request mutations', () => {
       expect(newMr.title).toBe('mr');
       expect(newMr.active).toBeTruthy();
     });
+
+    it('keeps original data', () => {
+      const versions = ['change'];
+      const mergeRequest = localState.projects.abcproject.mergeRequests[1];
+
+      mergeRequest.versions = versions;
+
+      mutations.SET_MERGE_REQUEST(localState, {
+        projectPath: 'abcproject',
+        mergeRequestId: 1,
+        mergeRequest: {
+          title: ['change'],
+        },
+      });
+
+      expect(mergeRequest.title).toBe('mr');
+      expect(mergeRequest.versions).toEqual(versions);
+    });
   });
 
   describe('SET_MERGE_REQUEST_CHANGES', () => {