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 {
-    name: 'MRWidgetHeader',
-    directives: {
-      tooltip,
+export default {
+  name: 'MRWidgetHeader',
+  directives: {
+    tooltip,
+  },
+  components: {
+    icon,
+    clipboardButton,
+  },
+  props: {
+    mr: {
+      type: Object,
+      required: true,
     },
-    components: {
-      icon,
-      clipboardButton,
+  },
+  computed: {
+    shouldShowCommitsBehindText() {
+      return this.mr.divergedCommitsCount > 0;
     },
-    props: {
-      mr: {
-        type: Object,
-        required: true,
-      },
+    commitsText() {
+      return n__('%d commit behind', '%d commits behind', this.mr.divergedCommitsCount);
     },
-    computed: {
-      shouldShowCommitsBehindText() {
-        return this.mr.divergedCommitsCount > 0;
-      },
-      commitsText() {
-        return n__('%d commit behind', '%d commits behind', this.mr.divergedCommitsCount);
-      },
-      branchNameClipboardData() {
-        // This supports code in app/assets/javascripts/copy_to_clipboard.js that
-        // works around ClipboardJS limitations to allow the context-specific
-        // copy/pasting of plain text or GFM.
-        return JSON.stringify({
-          text: this.mr.sourceBranch,
-          gfm: `\`${this.mr.sourceBranch}\``,
-        });
-      },
-      isSourceBranchLong() {
-        return this.isBranchTitleLong(this.mr.sourceBranch);
-      },
-      isTargetBranchLong() {
-        return this.isBranchTitleLong(this.mr.targetBranch);
-      },
+    branchNameClipboardData() {
+      // This supports code in app/assets/javascripts/copy_to_clipboard.js that
+      // works around ClipboardJS limitations to allow the context-specific
+      // copy/pasting of plain text or GFM.
+      return JSON.stringify({
+        text: this.mr.sourceBranch,
+        gfm: `\`${this.mr.sourceBranch}\``,
+      });
     },
-    methods: {
-      isBranchTitleLong(branchTitle) {
-        return branchTitle.length > 32;
-      },
+    isSourceBranchLong() {
+      return this.isBranchTitleLong(this.mr.sourceBranch);
     },
-  };
+    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"