Fixed alignment of merge request tabs and system header

Fixes the alignment of the tabs, system header and also
the performance bar.

Closes https://gitlab.com/gitlab-org/gitlab/-/issues/218300

app/assets/stylesheets/pages/diff.scss

@@ -38,6 +38,14 @@
       top: $mr-file-header-top;
       z-index: 120;
 
+      .with-system-header & {
+        top: $mr-file-header-top + $system-header-height;
+      }
+
+      .with-system-header.with-performance-bar & {
+        top: $mr-file-header-top + $system-header-height + $performance-bar-height;
+      }
+
       &::before {
         content: '';
         position: absolute;
@@ -1078,6 +1086,14 @@ table.code {
   max-height: calc(100vh - #{$top-pos});
   z-index: 202;
 
+  .with-system-header & {
+    top: $top-pos + $system-header-height;
+  }
+
+  .with-system-header.with-performance-bar & {
+    top: $top-pos + $system-header-height + $performance-bar-height;
+  }
+
   .with-performance-bar & {
     $performance-bar-top-pos: $performance-bar-height + $top-pos;
     top: $performance-bar-top-pos;

app/assets/stylesheets/pages/merge_requests.scss

@@ -771,6 +771,14 @@ $mr-widget-min-height: 69px;
     position: sticky;
     top: $header-height + $mr-tabs-height;
 
+    .with-system-header & {
+      top: $header-height + $mr-tabs-height + $system-header-height;
+    }
+
+    .with-system-header.with-performance-bar & {
+      top: $header-height + $mr-tabs-height + $system-header-height + $performance-bar-height;
+    }
+
     .mr-version-menus-container {
       flex-wrap: nowrap;
     }
@@ -788,6 +796,14 @@ $mr-widget-min-height: 69px;
   background-color: $white;
   border-bottom: 1px solid $border-color;
 
+  .with-system-header & {
+    top: $header-height + $system-header-height;
+  }
+
+  .with-system-header.with-performance-bar & {
+    top: $header-height + $system-header-height + $performance-bar-height;
+  }
+
   @include media-breakpoint-up(sm) {
     position: -webkit-sticky;
     position: sticky;

changelogs/unreleased/ph-218300-fixedSystemHeaderOnDiffs.yml

+---
+title: Fixed merge request tabs overlapping with system header
+merge_request:
+author:
+type: fixed

ee/app/helpers/ee/application_helper.rb

@@ -74,13 +74,6 @@ module EE
       ::Gitlab.config.smartcard.client_certificate_required_port
     end
 
-    def page_class
-      class_names = super
-      class_names += system_message_class
-
-      class_names
-    end
-
     override :autocomplete_data_sources
     def autocomplete_data_sources(object, noteable_type)
       return {} unless object && noteable_type