Fixes diff file tree list not be resizable

Closes https://gitlab.com/gitlab-org/gitlab/-/issues/346109

app/assets/stylesheets/page_bundles/merge_requests.scss

@@ -45,7 +45,7 @@
   top: calc(#{$top-pos} + var(--system-header-height, 0px) + var(--performance-bar-height, 0px));
   // stylelint-disable-next-line length-zero-no-unit
   max-height: calc(100vh - #{$top-pos} - var(--system-header-height, 0px) - var(--performance-bar-height, 0px) - var(--review-bar-height, 0px));
-  z-index: 202;
+  z-index: 205;
 
   .drag-handle {
     bottom: 16px;