Merge branch 'ph/fixEmojiPickerVirtualScrolling' into 'master'

Fixes emoji picker being behind diff tree list

See merge request gitlab-org/gitlab!65841

app/assets/stylesheets/page_bundles/merge_requests.scss

@@ -7,6 +7,7 @@
 .diff-files-holder {
   flex: 1;
   min-width: 0;
+  z-index: 203;
 
   .vue-recycle-scroller__item-wrapper {
     overflow: visible;