Expand collapsed discussion when replying

Closes https://gitlab.com/gitlab-org/gitlab/-/issues/235529

app/assets/javascripts/notes/components/noteable_discussion.vue

@@ -149,9 +149,14 @@ export default {
       'removePlaceholderNotes',
       'toggleResolveNote',
       'removeConvertedDiscussion',
+      'expandDiscussion',
     ]),
     showReplyForm() {
       this.isReplying = true;
+
+      if (!this.discussion.expanded) {
+        this.expandDiscussion({ discussionId: this.discussion.id });
+      }
     },
     cancelReplyForm(shouldConfirm, isDirty) {
       if (shouldConfirm && isDirty) {

changelogs/unreleased/ph-235529-replyExpandsCollapsedDiscussion.yml

+---
+title: Fixed discussion not expanding when replying to a collapsed discussion
+merge_request: 39571
+author:
+type: fixed

spec/frontend/notes/components/noteable_discussion_spec.js

@@ -89,6 +89,23 @@ describe('noteable_discussion component', () => {
         });
     });
 
+    it('should expand discussion', async () => {
+      const expandDiscussion = jest.fn();
+      const discussion = { ...discussionMock };
+      discussion.expanded = false;
+
+      wrapper.setProps({ discussion });
+      wrapper.setMethods({ expandDiscussion });
+
+      await wrapper.vm.$nextTick();
+
+      wrapper.vm.showReplyForm();
+
+      await wrapper.vm.$nextTick();
+
+      expect(expandDiscussion).toHaveBeenCalledWith({ discussionId: discussion.id });
+    });
+
     it('does not render jump to thread button', () => {
       expect(wrapper.find('*[data-original-title="Jump to next unresolved thread"]').exists()).toBe(
         false,