Remove stickyMonitor from diff_file_head.vue

In this scenario stickyMonitor only add the class `is-stuck`
to diff_file_header. On large diffs this almost doubles the
render time.

All the `is-stuck` class does is set border-radiust to 0.
I believe the performance impact is not worth the trade
off.

app/assets/javascripts/diffs/components/diff_file_header.vue

@@ -2,7 +2,7 @@
 import _ from 'underscore';
 import { mapActions, mapGetters } from 'vuex';
 import { GlButton, GlTooltipDirective, GlTooltip, GlLoadingIcon } from '@gitlab/ui';
-import { polyfillSticky, stickyMonitor } from '~/lib/utils/sticky';
+import { polyfillSticky } from '~/lib/utils/sticky';
 import ClipboardButton from '~/vue_shared/components/clipboard_button.vue';
 import Icon from '~/vue_shared/components/icon.vue';
 import FileIcon from '~/vue_shared/components/file_icon.vue';
@@ -11,7 +11,7 @@ import { __, s__, sprintf } from '~/locale';
 import { diffViewerModes } from '~/ide/constants';
 import EditButton from './edit_button.vue';
 import DiffStats from './diff_stats.vue';
-import { scrollToElement, contentTop } from '~/lib/utils/common_utils';
+import { scrollToElement } from '~/lib/utils/common_utils';
 
 export default {
   components: {
@@ -127,8 +127,6 @@ export default {
   },
   mounted() {
     polyfillSticky(this.$refs.header);
-    const fileHeaderHeight = this.$refs.header.clientHeight;
-    stickyMonitor(this.$refs.header, contentTop() - fileHeaderHeight - 1, false);
   },
   methods: {
     ...mapActions('diffs', [

app/assets/stylesheets/framework/files.scss

@@ -334,10 +334,6 @@ span.idiff {
     padding: $gl-padding-8 $gl-padding;
     margin: 0;
     border-radius: $border-radius-default $border-radius-default 0 0;
-
-    &.is-stuck {
-      border-radius: 0;
-    }
   }
 
   .file-header-content {

app/assets/stylesheets/pages/diff.scss

@@ -10,6 +10,7 @@
   .file-title-flex-parent {
     border-top-left-radius: $border-radius-default;
     border-top-right-radius: $border-radius-default;
+    box-shadow: 0 -2px 0 0 var(--white);
     cursor: pointer;
 
     @media (min-width: map-get($grid-breakpoints, md)) {