Don't show MLC form on thread replies

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

@@ -147,10 +147,10 @@ export default {
       return getEndLineNumber(this.lineRange);
     },
     showMultiLineComment() {
-      if (!this.glFeatures.multilineComments) return false;
+      if (!this.glFeatures.multilineComments || !this.discussionRoot) return false;
       if (this.isEditing) return true;
 
-      return this.line && this.discussionRoot && this.startLineNumber !== this.endLineNumber;
+      return this.line && this.startLineNumber !== this.endLineNumber;
     },
     commentLineOptions() {
       if (!this.diffFile || !this.line) return [];

changelogs/unreleased/jdb-fix-multiline-comment-form-reply.yml

+---
+title: Fix showing MLC form on replies
+merge_request: 37139
+author:
+type: fixed

spec/frontend/notes/components/noteable_note_spec.js

@@ -34,7 +34,13 @@ describe('issue_note', () => {
         note,
       },
       localVue,
-      stubs: ['note-header', 'user-avatar-link', 'note-actions', 'note-body'],
+      stubs: [
+        'note-header',
+        'user-avatar-link',
+        'note-actions',
+        'note-body',
+        'multiline-comment-form',
+      ],
     });
   });
 
@@ -77,6 +83,24 @@ describe('issue_note', () => {
       });
     });
 
+    it('should render multiline comment if editing discussion root', () => {
+      wrapper.setProps({ discussionRoot: true });
+      wrapper.vm.isEditing = true;
+
+      return wrapper.vm.$nextTick().then(() => {
+        expect(findMultilineComment().exists()).toBe(true);
+      });
+    });
+
+    it('should not render multiline comment form unless it is the discussion root', () => {
+      wrapper.setProps({ discussionRoot: false });
+      wrapper.vm.isEditing = true;
+
+      return wrapper.vm.$nextTick().then(() => {
+        expect(findMultilineComment().exists()).toBe(false);
+      });
+    });
+
     it('should not render if has single line comment', () => {
       const position = {
         line_range: {