Fixed changed files dropdown not being shown

Closes #36479

app/assets/stylesheets/pages/diff.scss

@@ -560,9 +560,13 @@
 }
 
 .diff-files-changed {
+  .inline-parallel-buttons {
+    position: relative;
+    z-index: 1;
+  }
+
   .commit-stat-summary {
     @include new-style-dropdown;
-    z-index: -1;
 
     @media (min-width: $screen-sm-min) {
       margin-left: -$gl-padding;

app/views/projects/diffs/_diffs.html.haml

@@ -6,7 +6,7 @@
 
 .content-block.oneline-block.files-changed.diff-files-changed.js-diff-files-changed{ class: ("diff-files-changed-merge-request" if merge_request) }
   .files-changed-inner
-    .inline-parallel-buttons
+    .inline-parallel-buttons.hidden-xs.hidden-sm
       - if !diffs_expanded? && diff_files.any? { |diff_file| diff_file.collapsed? }
         = link_to 'Expand all', url_for(params.merge(expanded: 1, format: nil)), class: 'btn btn-default'
       - if show_whitespace_toggle

app/views/projects/diffs/_stats.html.haml

@@ -10,7 +10,7 @@
     %strong.cgreen #{sum_added_lines} additions
     and
     %strong.cred #{sum_removed_lines} deletions
-  .diff-stats-additions-deletions-collapsed.pull-right{ "aria-hidden": "true", "aria-describedby": "diff-stats" }
+  .diff-stats-additions-deletions-collapsed.pull-right.hidden-xs.hidden-sm{ "aria-hidden": "true", "aria-describedby": "diff-stats" }
     %strong.cgreen<
       +#{sum_added_lines}
     %strong.cred<