Allow usage of `monaco-editor` in Jest

Partial fix that allows usage of `monaco-editor` in Jest and not make
the tests outright fail.

jest.config.js

@@ -35,7 +35,7 @@ const moduleNameMapper = {
   '^ee_else_ce(/.*)$': '<rootDir>/app/assets/javascripts$1',
   '^helpers(/.*)$': '<rootDir>/spec/frontend/helpers$1',
   '^vendor(/.*)$': '<rootDir>/vendor/assets/javascripts$1',
-  '\\.(jpg|jpeg|png|svg)$': '<rootDir>/spec/frontend/__mocks__/file_mock.js',
+  '\\.(jpg|jpeg|png|svg|css)$': '<rootDir>/spec/frontend/__mocks__/file_mock.js',
   'emojis(/.*).json': '<rootDir>/fixtures/emojis$1.json',
   '^spec/test_constants$': '<rootDir>/spec/frontend/helpers/test_constants',
   '^jest/(.*)$': '<rootDir>/spec/frontend/$1',
@@ -82,7 +82,7 @@ module.exports = {
     '^.+\\.js$': 'babel-jest',
     '^.+\\.vue$': 'vue-jest',
   },
-  transformIgnorePatterns: ['node_modules/(?!(@gitlab/ui|bootstrap-vue|three)/)'],
+  transformIgnorePatterns: ['node_modules/(?!(@gitlab/ui|bootstrap-vue|three|monaco-editor)/)'],
   timers: 'fake',
   testEnvironment: '<rootDir>/spec/frontend/environment.js',
   testEnvironmentOptions: {

spec/frontend/__mocks__/monaco-editor/index.js

+// NOTE:
+// These imports are pulled from 'monaco-editor/esm/vs/editor/editor.main.js'
+// We don't want to include 'monaco-editor/esm/vs/editor/edcore' because it causes
+// lots of compatability issues with Jest
+// Issue: https://gitlab.com/gitlab-org/gitlab/-/issues/209863
+import 'monaco-editor/esm/vs/language/typescript/monaco.contribution';
+import 'monaco-editor/esm/vs/language/css/monaco.contribution';
+import 'monaco-editor/esm/vs/language/json/monaco.contribution';
+import 'monaco-editor/esm/vs/language/html/monaco.contribution';
+import 'monaco-editor/esm/vs/basic-languages/monaco.contribution';
+
+export * from 'monaco-editor/esm/vs/editor/editor.api';
+export default global.monaco;