Merge branch 'fix-diff-comment-alignment' into 'master'

Fix diff comment alignment for parallel code additions

Closes #47861

See merge request gitlab-org/gitlab-ce!20006

app/assets/stylesheets/pages/diff.scss

@@ -65,6 +65,7 @@
 
       .diff-line-num {
         width: 50px;
+        position: relative;
 
         a {
           transition: none;

app/assets/stylesheets/pages/notes.scss

@@ -632,20 +632,18 @@ ul.notes {
 .line_holder .is-over:not(.no-comment-btn) {
   .add-diff-note {
     opacity: 1;
+    z-index: 101;
   }
 }
 
 .add-diff-note {
   @include btn-comment-icon;
   opacity: 0;
-  margin-top: -2px;
   margin-left: -55px;
   position: absolute;
+  top: 50%;
+  transform: translateY(-50%);
   z-index: 10;
-
-  .new & {
-    margin-top: -10px;
-  }
 }
 
 .discussion-body,