Disable MR check out button when source branch is deleted

After source branch deletion the Merge request `Check out branch` button is disabled.

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

@@ -76,6 +76,7 @@ export default {
         <a
           href="#modal_merge_info"
           data-toggle="modal"
+          :disabled="mr.sourceBranchRemoved"
           class="btn btn-sm inline">
           Check out branch
         </a>

changelogs/unreleased/42255-disable-mr-checkout-button-when-source-branch-deleted.yml

+---
+title: Disable MR check out button when source branch is deleted
+merge_request: 16631
+author: Jacopo Beschi @jacopo-beschi
+type: fixed

spec/javascripts/vue_mr_widget/components/mr_widget_header_spec.js

@@ -48,11 +48,15 @@ describe('MRWidgetHeader', () => {
   describe('template', () => {
     let vm;
     let el;
+    let mr;
     const sourceBranchPath = '/foo/bar/mr-widget-refactor';
-    const mr = {
+
+    beforeEach(() => {
+      mr = {
         divergedCommitsCount: 12,
         sourceBranch: 'mr-widget-refactor',
         sourceBranchLink: `<a href="${sourceBranchPath}">mr-widget-refactor</a>`,
+        sourceBranchRemoved: false,
         targetBranchPath: 'foo/bar/commits-path',
         targetBranchTreePath: 'foo/bar/tree/path',
         targetBranch: 'master',
@@ -61,7 +65,6 @@ describe('MRWidgetHeader', () => {
         plainDiffPath: '/mr/plainDiffPath',
       };
 
-    beforeEach(() => {
       vm = createComponent(mr);
       el = vm.$el;
     });
@@ -82,6 +85,8 @@ describe('MRWidgetHeader', () => {
       expect(el.textContent).toContain('Check out branch');
       expect(el.querySelectorAll('.dropdown li a')[0].getAttribute('href')).toEqual(mr.emailPatchesPath);
       expect(el.querySelectorAll('.dropdown li a')[1].getAttribute('href')).toEqual(mr.plainDiffPath);
+
+      expect(el.querySelector('a[href="#modal_merge_info"]').getAttribute('disabled')).toBeNull();
     });
 
     it('should not have right action links if the MR state is not open', (done) => {
@@ -101,5 +106,16 @@ describe('MRWidgetHeader', () => {
         done();
       });
     });
+
+    it('should disable check out branch button if source branch has been removed', (done) => {
+      vm.mr.sourceBranchRemoved = true;
+
+      Vue.nextTick()
+        .then(() => {
+          expect(el.querySelector('a[href="#modal_merge_info"]').getAttribute('disabled')).toBe('disabled');
+          done();
+        })
+        .catch(done.fail);
+    });
   });
 });