Re added The Web IDE Button

app/assets/javascripts/vue_merge_request_widget/components/mr_widget_header.vue

 <script>
-  import tooltip from '~/vue_shared/directives/tooltip';
-  import { n__ } from '~/locale';
-  import icon from '~/vue_shared/components/icon.vue';
-  import clipboardButton from '~/vue_shared/components/clipboard_button.vue';
+import tooltip from '~/vue_shared/directives/tooltip';
+import { n__ } from '~/locale';
+import icon from '~/vue_shared/components/icon.vue';
+import clipboardButton from '~/vue_shared/components/clipboard_button.vue';
 
-  export default {
+export default {
   name: 'MRWidgetHeader',
   directives: {
     tooltip,
@@ -41,13 +41,16 @@
     isTargetBranchLong() {
       return this.isBranchTitleLong(this.mr.targetBranch);
     },
+    webIdePath() {
+      return `${gon.relative_url_root}/-/ide/project${this.mr.statusPath.replace('.json', '')}`;
+    },
   },
   methods: {
     isBranchTitleLong(branchTitle) {
       return branchTitle.length > 32;
     },
   },
-  };
+};
 </script>
 <template>
   <div class="mr-source-target">
@@ -96,6 +99,14 @@
     </div>
 
     <div v-if="mr.isOpen">
+      <a
+        :disabled="mr.sourceBranchRemoved"
+        :href="webIdePath"
+        class="btn btn-sm btn-default inline js-web-ide"
+        type="button"
+      >
+        {{ s__("mrWidget|Open in Web IDE") }}
+      </a>
       <button
         data-target="#modal_merge_info"
         data-toggle="modal"