Only show message when MR is open

This commit fixes the bug that was causing the "target branch has
"advanced" error message to display after an MR was closed.

app/assets/javascripts/vue_merge_request_widget/mr_widget_options.vue

@@ -119,7 +119,8 @@ export default {
     },
     showTargetBranchAdvancedError() {
       return Boolean(
-        this.mr.pipeline &&
+        this.mr.isOpen &&
+          this.mr.pipeline &&
           this.mr.pipeline.target_sha &&
           this.mr.pipeline.target_sha !== this.mr.targetBranchSha,
       );

changelogs/unreleased/60808-only-show-target-branch-advanced-error-before-merge.yml

+---
+title: Only show the "target branch has advanced" message when the merge request is
+  open
+merge_request: 27588
+author:
+type: fixed

spec/javascripts/vue_mr_widget/mr_widget_options_spec.js

@@ -228,6 +228,7 @@ describe('mrWidgetOptions', () => {
     describe('showTargetBranchAdvancedError', () => {
       describe(`when the pipeline's target_sha property doesn't exist`, () => {
         beforeEach(done => {
+          Vue.set(vm.mr, 'isOpen', true);
           Vue.set(vm.mr.pipeline, 'target_sha', undefined);
           Vue.set(vm.mr, 'targetBranchSha', 'abcd');
           vm.$nextTick(done);
@@ -240,6 +241,7 @@ describe('mrWidgetOptions', () => {
 
       describe(`when the pipeline's target_sha matches the target branch's sha`, () => {
         beforeEach(done => {
+          Vue.set(vm.mr, 'isOpen', true);
           Vue.set(vm.mr.pipeline, 'target_sha', 'abcd');
           Vue.set(vm.mr, 'targetBranchSha', 'abcd');
           vm.$nextTick(done);
@@ -250,8 +252,22 @@ describe('mrWidgetOptions', () => {
         });
       });
 
+      describe(`when the merge request is not open`, () => {
+        beforeEach(done => {
+          Vue.set(vm.mr, 'isOpen', false);
+          Vue.set(vm.mr.pipeline, 'target_sha', 'abcd');
+          Vue.set(vm.mr, 'targetBranchSha', 'bcde');
+          vm.$nextTick(done);
+        });
+
+        it('should be false', () => {
+          expect(vm.showTargetBranchAdvancedError).toEqual(false);
+        });
+      });
+
       describe(`when the pipeline's target_sha does not match the target branch's sha`, () => {
         beforeEach(done => {
+          Vue.set(vm.mr, 'isOpen', true);
           Vue.set(vm.mr.pipeline, 'target_sha', 'abcd');
           Vue.set(vm.mr, 'targetBranchSha', 'bcde');
           vm.$nextTick(done);